D.P.
When my daughter was about that age she started having trouble sleeping. She too would just start crying with one reason or another like your daughter. It seemed that she was starting to show signs of puberty just like it stated in her American Girl book about her emotions and body changes.
About 3 nights of this I decided to take a different approach. Instead of cuddling with her to fall asleep, I sat with her in her room and asked her if she could make a list for me of everything that has been stressing her out lately. I said nothing you put on it is wrong. It could be as big as a girl bullying her at school, your afraid of failing math or you wish you had a new shirt.
The next morning she gave me her list. They ranged from afraid to fail math to wanting a new pair or shoes. We talked about each one and seriously, she felt better. I think part of it was that she had so many things going on she couldn't pin point it. Talking about it helped and having me listen made her happy. I also suggested she re-read her American Girl Book.
Hope that helps!