I have been reading this forum for years and this post made me come out of hiding. I have a DD who developed this problem at age 9 while trying to perfectly model a "healthy" lifestyle. She became dangerously underweight and it changed her personality and behavior. If you are seeing any of those red flags, please see a pediatrician right away. Early intervention is key.
Also, it is time to start sharing conversations about body changes that are normal and something that every girl goes through. Puberty takes alot of growth and calories, especially for those brainy, active high-achievers. Another thing to watch is the messages received at school and through the media about what is appropriate to eat. What is right for an average-sized/averagely active child is not okay for a thin and/or athletic child. Chocolate milk and ice cream are needed in my home. Kids this age still need around 30% of their calories from fats.
I'm ecstatic to share that my daughter fought and won this battle, grew 8 inches already and is thriving at her middle school. I am not trying to scare anyone, but just wanted to share that sometimes serious problems can develop.