Little Marble Oscar has a pot belly. Is he sick?

Hello all. I have a litte Marble Oscar who has a pot belly going on. I just noticed it around two days ago. So far, he doesn't have any problems swimming up, down or around, he still eats voraciously and will chase some of the other fish around. He isn't swimming erratically or upside down. When I walk by the tank, he comes up to the top like he's always done. On Saturday, I fed some live feeder goldfish. He helped himself to one. Could this be the reason for his little potbelly? He's only about 2.5 inches. Is my little fella sick or is he just gorging himself?

Thanks for the quick reply. I just walked back in and as usual, the little fella comes roaring up to the top of the tank where I'm standing. I notice that he has a HUGE sized piece of poop coming out of his backside! I'm thinking that he's still digesting. Since he's not acting sick and still wants to eat everything in sight, I'll keep an eye on him for a few more days.