Why did she insult my attire?

Once when we all went to a neighborhood pool with my in laws (their area) I wore a bikini at my husband's request. My stomach isn't extremely fat, but its not a 6 pack either. I have very large hips for someone barely 100 lbs and very petite. My MIL is also petite and the same size, but has large*you know what's*. When I wore the bikini she said "well, I on the other hand know that I have to wear a mom bikini when I am in public and as a mom I have to cover myself up. I dont have the thin figure that you do". She isn't fat, she is really the same size as I am. I have a child. But I was offended that she said this. She also has to always be saying how young I look and if I looked any younger than I would look like a child. whats her deal?