inflation
When the stomach is filled to excess with liquid or gas (or even a flexible object which nestles into the uterus or intestines) causing a round, outward expansion of the stomach.
If it's a penis or a solid object causing a bulge, use stomach bulge.
If caused by someone eating too much food, use full stomach.
Not to be confused with pregnant.
Not to be confused with the act of inflating, which is used for inflatable objects.