AIBU to be upset about silence and no explanation?

No. It'll make you feel worse than you do now.

That is your RSD though. People pleasing, going though every conversation to check for anomalies, driving yourself mad and then wanting reassurance and validation that people don't hate you.

Everything you're feeling is yours to sit with. It's not a fault, it's just the way you are. RSD is inherently self absorbed. It really looks like it sucks to live with.

Your friend doesn't have the spoons. You need to find another way to make yourself feel better.

Most people have voted that you're not being unreasonable and not in the wrong.
From reading the thread, some people have said you perhaps did the wrong thing but many people have also said you didn't do anything wrong (I was on this side).

But you've obviously decided to believe the people saying you were in the wrong here. I really don't think it's that black and white

Yeah I think you've done the wrong thing here, the timeline from her perspective isn't great;
Autumn bereaved
Christmas you asked her and she said she couldn't face it
She's not intimated she's even up for a walk and is quiet
You again ask if she can cat feed

I suspect she went "ofgs you know my situation and how I'm feeling, I'm clearly not on my feet/feeling OK at the moment, why have you asked again. I'll ignore and pop the key back because I can't face telling her again about how awful I'm feeling".

Personally, if she's a neighbour I'd have seen the Christmas "I'm not up for it" message and immediately grabbed some wine, or cake or flowers and popped them outside a door with a card saying I was always there for her as a friend. Then I'd have left it. Let her grieve, let her reach out if she needed me.

If you want to maintain the friendship, I'd get a card and flowers, write an apology saying sorry if she felt pushed to do the cat, you would hate her to feel that way, and totally understand she needs her space, message if you'd ever like to chat etc. Then let her be and properly leave it, no more messages cat related or otherwise. I suspect she'll come round when she's ready and she has processed some of her grief. You've just pushed too hard and probably made her feel 2 months is the amount of time it should have taken her to "go back to normal". That probably raised a whole load of feelings she is having and processing right now, unfortunately.
I wonder if it's worth you thinking about how this situation has occurred, and how it isn't the first time. Possibly there's a communication challenge there (maybe autism) or possibly you need to just think about others perspectives at times.

OP leave her alone. She's gave you your key back and blocked you. Take the hint and get on with your life.

The problem with Autism/ADHD/rsd is everything is made about the person who has it. Even now you're still thinking about how to make those confusing rejected feelings go away. It's yours to sit with. I get that it's probably awful to live with and I have every sympathy.

Any more contact from you will not be welcome and will probably make you feel worse. You need to protect yourself from feeling worse. A note won't help you, it'll just give you something else to stress about.

Get in touch with mates and have a get together or something.

Can I just clarify something @Lifeinblackandwhite

Did you ask at Christmas about April and then again recently?

Or was it 2 different occasions?
Christmas and April ?

I think if both were April she feels she said No already and feels annoyed / irritated you have asked again

If its 2 separate occasions then I think she feels its a big ask and quite honestly I agree.
Cat sitting is not her priority and I wouldnt ask someone who is grieving a favour like this
It comes across quite unfeeling and blunt

I do agree with this. A neighbour had a bereavement and I deliberately made a meal in a foil tin tray so they could just chuck it away afterwards.

But if you asked at Christmas time (end of December) and have asked again now (beginning of March) that's not even 3 months. Christmas and New Year are really hard for newly bereaved people, so really, she's had about 2 months of 'normal time'.

Often, 'no pressure' inadvertently puts more pressure on.

I think you should leave the ball in her court now and not leave a note. You have tried to reach out multiple times already, she’s getting your messages but choosing to not be in touch. She knows you care about her.

It sounds like a combination of you misreading her signals and her being overly sensitive due to her grief.
I personally wouldn’t have asked for a favour again until she was back to messaging or meeting for a walk/ coffee and more time had passed.
You haven’t done anything wrong or malicious so don’t be too hard on yourself it’s just clearly been taken badly by her during a difficult time.

I really don't think the neighbour is thinking that OP should do more. I think she wants to be left alone. It's quite clear since she hasn't taken OP up on any of her requests to mee up.

Doesn’t sound like they were close friends. Sounds like they’re acquaintances / neighbours. And OP kept texting asking to meet up when neighbour was bereaved.

Receiving requests to meet-up / do stuff etc is stressful and smothering when you’re going through a big bereavement or mental ill health. When you’re in a total mire there’s only so many times you can say “no thanks” to an acquaintance before you start feeling peed off at someone and wanting to step-back to preserve your mental energy for the essentials (family / kids / best friends).

When that person doesn’t get the hint but instead asks (again) if you can feed their cat…well, you pop their key discretely back and draw a line under things, having dodged a stressful bullet.

People coming to my door would be the WORST thing possible for me.
A text would be far far preferable.
Don't push your preferences on others, or judge if they dont behave in away that you have deemed appropriate.

Obviously more sensitive than you.
Pop back when you've been bereaved for 6 months.

You say she had a bereavement you “think” was in autumn. That doesn’t sound like you’re particularly close with this person.

In all of this messaging, have you ever just gone and knocked on her door to see how she is?

She sees you as a person who is using her for cat sitting. That’s why she has returned the key. If you want to genuinely be friends with her, no strings attached, no expectations of cat sitting, go round and apologise for upsetting her. No excuses, no reasons, just say sorry and offer some real, tangible help.

That's sad OP. I would knock on her door and get it out of her. Say you are confused and want to understand if you were offended her. Face to face best thing

You're not a bad person. The fact you made yourself available and offered social outings, etc (probably well out of your comfort zone) is not wrong.
It's well and good for NT people to say you didn't see the signs, you overstepped, etc, but we don't see things clearly like that. It's helpful to have things explained fully, even timescales, as they differ from person to person so much. It leaves too much room for doubt and as appears now, mistakes can be made due to lack of clear boundaries. Though your friend is grieving, it was probably hard to let you know all this with everything else they're processing too.
Please try not to feel bad. They're not wrong in needing space and maybe thought they've done the best thing by ghosting you, to save the hurt of a face to face ending. It's a shame, but it's not on you.
If you bump into one another, acknowledge them, but let them make the first move for engaging in conversation. That way they know you see them, but are allowing them space should they still want it. If they walk away, remember it's nothing you've done (the gut punch you feel will pass).
Hope you find a cat sitter and have some other friends to spend time with and lift you up, so you can see what a kind person you are.

Hi,
It sounds like your friend is grieving after their recent bereavement. There is no right or wrong way to grieve, some people seek company, some withdraw. Your friend has told you they are unable to commit to cat sitting and on practical terms returning the key allows you to give it to someone else.

In line, the friendship may reconnect but for now, let her be. If she wants to or is able to talk, she will.

So you wouldn't find it draining to have to interact with someone. Why does that mean everyone else must feel the same way? What a narrow world view, and how nasty to label someone's behaviour pathetic because you think they should behave the way you like.