My 18 year old is about 5"10, and app. 135 pounds. My 14 is 6'2 pushing 6'3, and weighs (I kid you not) 143-145 lbs (he needs to gain obviously).
Your daughter's weight sounds "off" for her age and height. I've seen girls in my son's peer groups with similar problems (so don't feel alone). I can't figure out what is going on with our girls. *Something* is happening.
I would get to an integrative physician and nutritionist, and figure out exactly what is going on (via blood work, stool testing, etc.). I would resolve to quit eating franken-food, even at home (no genetically modified, chemical laden food). I would endeavor to eat whole foods, preferably organic, as often as possible. I would do everything in my power to cut out processed sugar (even in healthy food). I would look into how much soy my kid is eating (phyto-estrogenic issues there) and talk to a professional about that.
The last thing I would do is make my daughter feel bad about what is happening. It is very likely that she cannot help it. Nor can you, without a great deal more knowledge. The mega food manufacturers have the deck stacked against us imho.
I would consider consulting with these professionals by myself first, so as to avoid any finger-pointing or blaming towards me or my child.
I would also investigate diabetes, hyperinsulinemia (sp?), and any other blood sugar issues.
Good luck.