Is this odd for an 11 year old?

I'm sorry to hear this, is there any chance she could be being bullied either at school or online? Is there also a chance that something could have happened and she is keeping it from you? The closed door situation seems like a safety thing, has someone threatened her at all?

She could be looking forward to secondary school at the same time as knowing that she will miss primary school.

How is she with her friends?

Are some friends going to a different school? That might be the sort of thing she hadn't thought through as an implication.

Starting period and didn't discuss it yet?

Bullying?

Struggling academically?

You'll need to talk with her. Maybe do gardening or another activity together to open some gentle space where she doesn't need to be looking at you to speak, and let her know you feel she has seemed sad recently.

Is she other phone a lot? If yes, are you looking at what she is doing?

If she has private access to the Internet, have you regularly and recently checked what she's been doing on there?

I'd check her phone if she has one check any social media apps she has like til tok snapchat Instagram especially any messages on them

You do need to be checking the phone at age 11, monitoring the message exchanges, what websites are being visited, is she making tic toks etc.

I don’t know the kind of things they were talking about. Anyway then she got upset because her friend didn’t text her back for about two hours with no warning. When she did text back, her friend apologized and said she has to hand her phone over to her parents for them to check messages. This didn’t really occur to me as DD is sensible and only has friends on there who she can speak to in real life too. I did wonder if maybe DD had written something which she didn’t expect an adult to be reading through?

Oh wow.
You have got to read your dc's messages at that age and probably until about 14/15 at least. Tell your dd that whatever messages she sends on the phone can be shared with anyone and everyone and that most responsible parents will read their dc's messages. Tell her to never ever post or share anything she wouldn't want her grandparents, headteacher or her whole school to read or find out. Kids that age LOVE taking screenshots and sharing them in the wider peer group to cause drama and falling outs, it's hugely fun to some kids.

What do you mean by grown up talk? Stuff around boys, crushes, relationships or politics and news or something else?

Edited for grammar

Tell your dd that whatever messages she sends on the phone can be shared with anyone and everyone and that most responsible parents will read their dc's messages.

This means that whatever she posts on WhatsApp, is likely to be read at least bu some of the class parents.

When you say "she is a joy to raise apart from the occasional outburst" what do you mean?
As in can you describe how she is a joy to raise and what do the outbursts look like?

My initial thought was check her phone. After that, I don't know. Perhaps her period is on the horizon and she's struggling to deal with the hormonal changes and emotions it's bringing? My DD struggled a bit before hers started.

She also spends most of her time either in her bedroom or out with her friends!

Maybe a mum and daughter day with some gentle (and not obvious) questioning and probing, might make her open up a bit?

It's very likely hormones kicking in op. But yes, check her phone.

You need to be checking her phone messages, I did this daily with DS until he was 13/14.

This.

OP, too many of us have thought our DC are ‘very sensible’ …

Check the phone regularly and hammer home the message that anything she writes could be seen by other parents and indeed by teachers.

As well as checking her phone messages, do you have parental controls so she can’t download apps without permission? Do you check what she has been looking at on safari, for example?

My DD is only 9, but I was speaking to a friend with older children about getting DD a phone in a year or so. My friend very strongly advised having the maximum controls on the phone to start with - checking messages, needing parental approval for downloading apps, limiting time on the phone, taking it away in the evenings - all stuff I’d never really thought about. It sounds as though your DD has no controls on her phone apart from restricting social media.

I’m very shocked you’re not checking her phone and let her have access to both WhatsApp and the web with no safeguards. She could be on chatrooms for a start and she can get on literally anything via safari, including web versions of apps and anyone could add her number to WhatsApp. She could be receiving all kinds of content from her classmates, or sending them images of herself and vice versa. It’s really quite basic internet safety and she is a child, she’s no way mature enough to handle this stuff totally unregulated.

I wonder if she has recently seen something or learned something about growing up that has distressed her - aka elements of sexuality, sexual acts, images she didn't understand.

I remember being around this age, and more streetwise kids taking great joy in sharing everything their older siblings had told them about sex and disturbing crimes, horror films and the suchlike.

I used to find it all really upsetting, and almost felt ashamed of what I now knew - when I was informed by two friends at a sleepover what 'a rapist did', I was traumatised for a long time.

Just a thought, as I think I probably behaved a bit like this when wrestling with these new, adult discoveries.

All the best.

Sounds like puberty to me.

Hormones
Argument with friends
Some unpleasantness online
She's worried about secondary school or leaving primary and her old routines (I know you said she was excited, but there's a difference between looking forward to something 2 years in advance and getting worried about it when it's actually about to happen)

I might actually suggest it's more likely to be the last.

It could be a romantic disappointment. I was like that at that age, for that reason.

Agree with the above re checking her phone.
My friend has a DD in year 7, and over the past year or two, has found countless messages on her phone to older boys, many of whom she doesn't even know, sexually explicit messages and images. Police and school have been involved as it's been so serious. Previous to this, the girl seemed very young for her age and we wouldn't have thought she was doing this.

Oh, and a lot of bullying and nasty messages from girls thought to be her friends were also found.
Not saying this is the case, but it's been eye-opening.

Could be bullying - also if it is bear in mind it can be really hard for them to speak up about it, it took us ages to work out that DS was being bullied, he gave every excuse under the sun until we finally twigged what was actually going on.