How can I support my 16-year-old who is struggling

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my child 16F, somethings going on with them but I don't know how to help them. They're consistently coming home from school early as they feel sick which I'm not questioning yet it's just a bit suspicious. They're behind on their school work and they've got their GCSE exams now really though she doesn't seem stressed about them and she got A*'s last year so I'm not concerned either. And I've heard her crying in her room for a couple of weeks now but she refuses to talk to me or her dad or go to the schools help team. And I'm just worried about her. She's had bad relationships with people in the past so it could be that but I'm not sure.
Do you have any suggestions on how I could help her or try to understand what she's going through?

Have you tried asking her?

What do you mean when you say she got A* last year is she is sitting her GCSE exams this year?

Yeah, shes in year 11. And in the majority of her exams she got A's and A*'s. We've tried talking to her but she shuts down and avoids the conversation and just ends up crying so we leave it though we can try pushing but I doubt that'll work

I've been through some of this, or shades of this with my five. It is a really tricky age emotionally, pressure, becoming an adult, etc. Firstly, don't take it personally, and don't stop letting her know that you are always there to talk to. Make it clear that no problem is too big and you are there for her, always and forever, but you are not going to nag her or be on her case Write it down if it helps and tell her you love her, and nothing isn't "sortable" and she can always talk to you.

Without invoking "snitching" which is the worst thing ever, is there someone that she trusts that she might open up to. Someone you could ask "let me know if there is a really big problem" without them telling you the details.

You could also make an appointment with the school on the absolute quiet, tell them you are worried about her, and talk it through with someone - it might not help, but it might.
And, although I don't think this applies here, I think it is true when people say "you are only as happy as your least happy child". Just on the experience front of this, I have been worried sick about mine, up all night etc. and then found that they were "absolutely fine" when out of my sights. Mine gained a bit of maturity at some point between 16-26, and realised it was not fair (nor convenient) to have their mother worrying about them, up all night etc..... But it takes time. Good luck, it is a difficult path to tread xx