I'm not sure what it is, but I can tell you that I am the same way. I exercise multiple times a day because it relieves anxiety and makes me feel better about my body. It's like I'm "emptied" the toxins from my body even if I haven't eaten anything. I think part of it is psychological (what you see) and part of it is physiological - letting go of water retention, muscles, etc. Possibly some dehydration? That can make you look different and make muscles appear when they normally wouldn't.

I'd be interested in what other's said or if it is a form of BDD.