My first thought was bloating which can be very serious but if the cat has no other symptoms and is eating and passing stool I'm now thinking roundworms. Of course, it's impossible to give you a definitive diagnosis without examining the animal, so I would suggest you have your vet give the cat a look.
If the cat hasn't used the litter box other than urinating its most likely constipation . Put a teaspoon of mineral oil on the cats food. You ultimately really need to visit your vet to assure not a blockage . Cats health goes down very quickly