it's very common! Some people simply fear any change, just that murky unknown of what will happen next. I think more common, though, is the sense of losing your protective shell. You feel that you are somehow getting revealed in new ways, and it's uncomfortable.

For people who have had childhood physical or sexual abuse, this is no small matter. The protection of fat was protection from the abuse-- "if I'm fat they'll leave me alone or I can fight back better." But even typical adolescent experiences of emerging sexuality and all the strange new troubles it brings can drive people to become large to avoid that whole mess.

A lot of us who have been both thin and fat feel very irritated at the changes in how we got treated thin...and so fat becomes a "prove you value me for myself" test. People give you a lot more credit and want to be your friend more and treat you as sexual more as you get thinner--that's the horrible truth. At one level it's nice...but at another it's enraging.

There are other possible reasons, too, but don't feel as if you're alone in the feelings, no matter what they are! I believe it's worth discussing with a counselor, especially if you aren't quite sure why.

HTH