She looked annoyed. Should I message her or not?

My girlfriend's great grandma died 2 days ago. She pmed me that day, she told me about it and was pretty sad. I kept asking her questions like how she died etc, and at some point she said ''I don't want to talk more about it hah''. So I said ''Silly are you better at least?'' , she said ''Yes yes'', then I said ''Okayy'', she read it but hasn't replied since then (been more than a day so far). Usually when conversations between us end she always says something. I don't think she is incredibly sad either since she uploaded stories on instagram etc and she didn't have many relations with her great grandma.

Should I message her now or not at all? I can't understand why she got annoyed (at least it looked like that), and why she read my message and didn't respond even though she has never done that ever before. What do you think? Should I wait for her message?

The conversation just seemed to come to a natural end. Nothing you've said here seems like she's annoyed at all.

You’re overthinking things
You sound young. She’s your girlfriend as you say so message her to find out how she is today. It’s not your place to decide whether or not she grieves/misses her gran because they weren’t close
She actually might be very sad inside or might not be. All you can do is be there for her
Ask her what she needs, offer to take her out/hangout.
Just be romantic and present I’m sure she’ll appreciate that

Yeah, it did end, but it's the first time she reads my message and doesn't reply to it. Usually when convo ends she revives it no matter what.

Well it looked like she is annoyed, I mean, I asked her nicely "Silly are you fine?" and she replied with "yeah yeah", in a cold way. I can't understand why, and also why she didn't want to talk about it. I just wanted to show to her that I care.

It's okay to not have anything to say after a while. She most likely just didn't have another topic to talk about and that's fine.

She simply responded by confirming that she is fine when you asked, and the way you've quoted her doesn't sound cold at all.

If you're worried you're going to have to meet with her and talk to her about it. But you are definitely over thinking this.