Agree with Ady- a vet visit is in order. Once the vet clears kitty of any health concerns, you might want to be sure the litter box is clean enough. Add a second litter box if you are only using one. Major rule is one cat, two boxes, one to pee in one to poop in. Get a larger box- I use those plastic slideout drawers that fit under your bed to put clothes in. They work great and the cats love them. Make sure the litter box is in a private enough area for the cat to use. But first and foremost take kitty to the vet to rule out any health problems.