Answer:
I believe she would have Anorexia, which is a condition in which someone feels the need to lose weight, even if they aren't inherently overweight.
Explanation:
I only know this because my family would tease that I might be anorexic just because I don't eat a lot and I'm a little thin. I know it's a bit insensitive, but they don't do it all the time or mean any harm by it, so...
Anyways, yeah, hope this helped :)