Sometimes people will say something negative about themselves because they're hoping you will say something positive, instead. I used to do the same thing. Whenever I was feeling insecure about my looks or weight, I would say something negative about myself, because my husband would tell me it wasn't true and would give me compliments that would boost my self-confidence. I don't do that anymore, because it's not a very healthy way to be confident in yourself, but it certainly was effective. Do you think maybe your girlfriend is just feeling insecure about herself? I would talk to her about that to see if maybe it's something she'd like to work on in herself. You can also try to give her honest compliments on a regular basis. It's hard for a girl (or guy!) to feel unattractive if her loved one is telling her twice a day that her shirt looks good on her, or he loves her laugh, or he thinks she's pretty.
I think it's because if dumb people commenting about every little change of appearance. Sadly we live in one of those places where people comment on these things all the time. They told her she is to thin before now they tell her she gained weight. There's no way to win with them, and sadly usually they are close family members that you can't just avoid.