I don't know why I feel this way?

i am 23 years old, female, 5'3 and 111 lbs. About a year and a half ago I weighed 100 lbs and now I feel like a whale. I'm so self concious about my body that I refuse to wear t shirts, tank tops, or shorts...only oversized sweatshirts. Sometimes I wear mens xtra large on down shirts because it makes me feel skinnier. I can't even look at myself in the mirror sometimes. But there are women who weigh more than me who I think are beautiful. What's wrong with me?