If your cat is a male, then he could have a urinary blockage which is an emergency, or it could be as simple as an infection
I agree with Pretty Kitty, watch your cat for straining, crying, urinating in weird places, or blood in the litter box.
Good luck
Staci
Could be the beginnign of UTI (Urinary Tract Infection), painful for kitties. Have you noticed if he cries or complains when he goes to the bathroom.