I KNOOOW how much that sucks!!! My mother has made a few remarks about my appearance too. We joined a gym together once, and we went to work out. I had some baggy track pants on, and on our way back I was walking ahead of her, and she said 'your butt is SO saggy!'. I was so humiliated, and I was on 16 or 17 at the time. Even though it was just my pants, it still hurt because her remark was intended at my body.

Sometimes the people you expect to support you most are the ones that hurt you the most. If she keeps making those remarks I wouldn't hold it past yourself to point out that she is being really nasty.