"You really should have had the conversation already" this!
Have you discussed ANYTHING sex ed related at all? Bodily autonomy?
I was a late starter but several relatives started around 9/10 and they're now 40+
Plus she may be experiencing other changes she is scared by and doesn't want to mention. I do not understand parents who leave this to the last possible minute.
Ask her what if anything she knows about how babies are made - i think it very likely she'll have a lot of misinformation you'll need to deal with first, I've worked in a voluntary capacity with children of this age - prepare to be shocked because her school friends have older brothers and sisters who discuss all sorts in probably quite a crude way, plus her friends parents will probably have been more open.
It'll also give you an idea of where her level of understanding is in terms of the words and phrases she uses. Be lead by her, keep it simple, be calm and ensure she understands it's nothing to be afraid of BUT it will probably be painful and she may feel "odd", moody etc. Be clear she can ask/tell you ANYTHING and you won't be shocked/cross.
I have a 17 year old dd and honestly without wishing to sound harsh you need to get over yourself. We have a very open relationship but I've seen/she's told me stuff that's gone on with her friends and it's a cliche but it's true - the ones with the more conservative parents are FAR more likely to have sex younger, have more partners, give in to pressure to do acts they don't really want to etc
Please DON'T leave it to JUST books and I'm sorry Juneau but he info in teen mags was AWFUL! She may have questions, worries, fears and need your reassurance plus it will give the impression you don't want to talk about it, that it's a thing that isn't talked about. Good quality books can work well as SUPPORTING material but shouldn't replace your parenting.
Herethereandeverywhere I have endo as do several relatives. I still didn't assume dd would BUT I did as she got closer to puberty say to her IF she experienced a lot of pain, bleeding she needed to tell me so we could go to the dr and get it looked into. Completely up to you if you say or not but I wonder if the reason for your difficulties was diagnosed/treated. It's important this isn't ignored as whatever the cause is could possibly lead to infertility or mc as in my case.