Needing advice

[con4_]

Well, this is about Nadia gaining about 30 pounds since summer. She was a chubby kid( ages 10 - 12), through no fault of her own. I saw her suffer and cry in dressing rooms. Then.... when she turned 13, she grew a few inches and thinned out beautifully.  Then, onto high school and she began to put on a few pounds.... not too bad but since I know what it is like to be an over weight teen, I worried. Then, this summer, her best friends are driving and I swear... they took her to Taco Bell every day! I can see now that she seeks comfort in food. She used to eat healthy but now she eats junk food and if I don't keep any at home, there is always a friend who will drive her to get ice cream or tacos or whatever...... They love to eat pizza in bed. She used to be athletic but as much as we insisted, she did not pursue basket ball in HS, after all. All she does is sit at the computer! She had a couple of heartbreaks, one in January where I saw before my very eyes, how she is trying to punish herself with food. Surely, she sees what is happening. Why does she continue to eat this way? None of her clothes fit her. What she wears is too tight and makes her look even worse. But she won't go buy the right size.When I try to talk to her about it , she gets very upset and says that my interfering just makes her want to over eat more! Good grief! I don't know what to do! She is only 16. I am sure that if she just started walking and eating better, she would lose this weight soon enough.
I know that she is not happy with this but something is going on.....
Am I being a vain and controlling mother?
Any suggestions?