don't go by what the charts say. I have a 13 year old as well and about a year ago she had some lactose problems and we were at the Dr's a lot. Well this Dr took it on herself to say that my daughter was underweight at 5'4" weighing 109 lbs. She started asking her if she thought she was fat (the girl had never given it a thought!) With my weight issue we never discussed her because she is thin like her dad. Well she is going through puberty and just had her physical and is now 5'7" and weighs in at 134. The nurse weighed her twice because we couldn't figure out were she puts it!!! It is muscle. The girl is active and plays competative soccer 3X per week between practice and games and everything else she does in her life.
This is her body type - she wears a size 1 or a size 3 she is not either underweight or overweight!!!

Sorry for the rant! Look at the body and see if she is happy in her own skin!!!