She is getting her "fat" thoughts from her friends, from the media and from society. In fact, I am surprised she did not mention something about her weight sooner. I forget the exact statistic but the majority or pre-teens and teens think they are overweight and are dieting in some way or another. The best thing you can do is to consistently tell your daughter that she looks just right. Teach her healthy eating and exercise habits. Unfortunately, she will probably never be happy with how she looks but you can at least try to minimize her negative thoughts about her body.