If it looks like a thumb poking out of your skin, it happened to me once, it turned out to be, and I kid you not, cat scratch fever. It is a real thing, you can only get it once and antibiotics cleared it right up. That doesn't mean that is what you have.
It could be a hernia. This should be seen by a doctor immediately as it will only worsen with time. Its a tear in the abdominal wall and its your guts or innards trying to poke through. Very painful and it does not heal on its own. It's a little smaller than a chickens egg and can be discolored or bruised in appearance.