AIBU to worry about my 15-year-old missing 2-3 period?

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looking fur advice
my 15 yo started her period at 10 has been largely regular since around 11-12 however no period since 15th April
she’s not sexually active I’m really sure of that
she has gained a stone over past year but she’s not overweight

coincidentally I’m 45 and have not had a period for 45 days when I usually have them every 27-32. Days i was also very regular! I can’t work it out? Anyone had similar

When I left home for the first time, to go to college, my periods stopped, and I was absolutely certain I was not pregnant, but it was worrying that I also felt so terribly tired. I guess I was also mildly stressed by living away from home and fending for myself more, and the academic demands were (not surprisingly) a step up on my school studies, although I was happy in my new setting.
(I have also known the reverse during my teaching career, where the "change of air" has precipitated the first period on School Journey or Guide camp!)
Luckily one of my fellow students in the hall of residence, who had previously had the same problem, made a suggestion which proved to be on track - I was anaemic.
Meals were served in sittings, and as first years, we got the last one, by which time the vegetables had been languishing in bain-marie containers long enough for all the Vitamin C to be long gone, and I learned from my new friend that if you lack Vitamin C, your body will not absorb iron efficiently.
The solution was a mug of cocoa daily (real cocoa, not drinking chocolate!) and a portion of fresh fruit. I was lucky that, for me, the solution was so simple.
It may be that your daughter is anaemic, especially if her periods can be heavy when they happen, it may be that there is some other more complex medical cause, but PP's are quite right that your GP will want to eliminate the obvious possible cause before embarking on more intrusive tests.
You could put it to her that it would save time to try the simple stuff at home before seeking advice.
I hope you are able to sort this out easily.

Same here
But the Op says her daughter is largely regular
I was never regular.

Speaking from experience from 20 years ago, a GP will send her away saying 'go and buy a pregnancy test and come back if it's negative'.

I was not impressed at the way they treated her - she wasn't pregnant, wasn't sexually active and even if she had been, by treating her like that, they stood a very good chance of missing girls who were being abused/could end up concealing a birth.

Asking a person who has no sexual experience to take a pregnancy test is insulting and humiliating. Yes of course she could be lying but it's as likely, or more so, that she is not. As her mum you have to believe her. To insist that she does this regardless of what she tells you will show her you don't trust her and also that you condone her being sexually active at that age, neither of which you'd want. Bring her to the Dr, if Dr insists on her taking a pg test at home then that's different. You can explain to her it's standard procedure to rule it out before they move to the next investigation and no reflection on whether they believe her or on what they think of her.

That's a waste of an appointment and could delay her getting diagnosed and treated (or if it does turn out that she is pregnant, an additional wait for dating scan/pregnancy care/supplements and a reduction in her potential to make choices).

Well so be it. I think we should believe what our teenage daughters tell us so they know they have a safe space when they need to share. If a Dr insists on a pregnancy test despite being told a pregnancy test isn't required that's on the Dr. Maybe a compromise would be to purchase one and bring it in a bag, explaining to DD is might be a standard requirement then do a test there as proof so the Dr will investigate.

One of my Dds was fairly regular when she started then went nearly a year with no period & it turned out to be PCOS.

We sought advice from the GP after a few months, then again & were told it was normal, we went private when it was getting towards a year.

Periods in teens can be all over the place but once you have ruled out pregnancy then I would consider exploring PCOS.
The symptoms vary wildly between women which is why lots of women don't actually realise they have it until perhaps they are trying to conceive.