thapa kaaji, if you are eating well and doing regular exercises, you shouldn't be gaining weight. like bittertruth said, there might be some problem with your endocrine system. since you said that you'r belly is still popped out (aka fat accumulation), i'd guess there's something funny going on in your liver, pancreas, or thyroid glands. eat a healthy and balanced diet for few weeks (in case you are not doing that now) and see if things change. if not, go see a doc or an endocrinologist.
btw, if you drink, quit it.