Now I've read your updates and realised you declined the Christmas dinner I think this could be the issue. I think there's a big difference between accepting someone's invitation to spend Christmas Day with them and just calling in for an hour late afternoon. She invited you to spend Christmas at her house, likely because she did want to 'take your under her wing' a bit and I'm guessing for her part of this was her feeling good about looking after you, possibly to talk about to other people, but probably just a feeling that she is a good person. She wanted you to spend the full day there, she wanted to take care of you, she'd even thought about transport for you, I think the offer was sincere. You declined this offer and instead said you'd call in afterwards with your own (specialty) food. This may have unintentionally caused some offence - she wanted to host you to be generous but now it's reversed, you've declined her generosity and instead you're turning up afterwards, not clear what time and taking your own gifts and food.
Sorry big reference, @pcmcgregor!
I made it quite clear in the email that I'd be more than happy to spend Christmas lunch with her, but I know that this year has been hard with Covid and she might want time to spend time with her family (as such bringing in the "popping in" plan) which I left quite open-ended so she could say "No, don't worry about it at all, come anyway" or "Yes, popping in would be easier". I figured it would be more abrasive to show up and make her cook me a plate of food if she just meant it as a social nicety. Maybe that was my mistake, but I'm not great at reading this many layers between the lines. I feel like I'm in the Matrix 
I wish I could just show the email for clarity purposes but that would be VERY outing 
She offered something that didn't suit you, but instead of declining you've altered it to something that you would prefer. She probably thinks that she was doing you a favour by not leaving you on your own all day maybe thinks that you are being rude? A bit-well I'll be OK for most of the day, but I'll turn up if I get fed up with being on my own?
I'm trying to make things the most convenient and fun for everyone, I didn't realise offering options that she could potentially take (therefore sparing her the awkward "No don't come for lunch but hang out instead" conversation) could be misconstrued that way. If this is how people usually communicate I feel as though I may find myself in this situation more frequently than preferable!