D.,
This is very normal, especially if she grew very fast. The same thing happened to me about the same age. I grew to 36C in the length of one summer. I got stretch marks, actually quite a few. The good thing is, when I had my children I didn't get additional ones and by the time I was an adult they were all normal skin tone and are not really noticeable.
My advice to you is to not make a big deal about it as she is having to deal with enough changes as it is! If she has changed significantly over the summer, she may get teased at school, I did. But, hopefully, she has a good core group of friends and that won't be an issue. I'm just saying that with all the changes going on with her, she has a lot to deal with. Just be willing to talk if she needs to and don't get too excited about anything...at least outwardly. :)
I hope this helps. God Bless, ~J.