I'm not sure if this belongs here, but..

I'm 19, male, 5'8", and 143 lbs. My stomach is large and round, sort of like I'm always bloated; by round I mean it goes outwards in pretty much a spherical shape, sort of like a pregnant woman, though not as far out. It's a bit firm, though still somewhat soft. In comparison to the rest of my body fat, it looks strange. At first I thought I just had a lot of excessive fat built up around my stomach, but then I figured that if that was the case, it would sag/jiggle rather than being firm. If I lay down on my back and don't hold my stomach out, it falls in past my rib cage.

I've seen a doctor before, and he just told me that it was nothing to worry about and sent me off. I've also done quite a bit of googling, but I haven't been able to figure much out. Any idea what could be causing my stomach to expand like this?