Well, i would definatly talk to your doctor about this. But, maybe its a problem with a lack of exercise. Even walking can help you to loose weight. I would suggest talking with your doctor, and if they dont know what to do, see a dietitian or a body fitness expert. Who knows, maybe your one of those people with that rare illness who gains weight when they undereat, and looses weight when they overeat. Also, if you have previously had an eating disorder
(such as anorexia
or bulemia), it can cause your body to gain weight unexpectedly.