What possible medical reasons can cause unexplained weight gain?

I had Gastric Byp 3.5 yrs ago. In the last 6 mo. I have been steadily gaining weight. I eat right. I don't eat anything that I shouldn't. I have even cut out things that I thought MIGHT cause problems, i.e potatoes and rice. I don't eat any wheat based products (I have an allergy to wheat) I stick to meat veggies, fruit, and some dairy. If I allow myself a treat, it is a sugarfree popsicle, or fudgesicle. I am very fustrated. I have gained about 40lbs and it is scaring the heck out of me. I would have to say the only bad thing that I do, is probably not get enough exercise. But even at that I am probably only eating between 1200-1500 tops in calories, but I have put on 4lbs in 2days. I am going to go insane if I can't find the answers. A friend suggested that maybe I am eating to much fruit. As far as the lack of exercise, I can understand maybe not losing, but the gain is what is so puzzling. This is why I am looking at maybe a medical reason. I did have my thyroid checked,its okay