It could be a reoccuring UTI or crystals in the urine - take a urine sample to your vet, or have them get one.

The longer the problem goes on the more likely it is to become a habit, never take the 'wait and see' approach when it comes to inappropriate elimination!

Cats sometime will snub a litter box that they've used faithfully in the past - All you ever wanted to know about litterboxes

Stress can cause a cat to stop using the litterbox - hence why my cat Cami has to live in a room by herself now.

Make sure to clean each pee spot with an enzyme cleaner - I have great luck with Nature's Miracle (I even use it to clean my garbage cans!)

Hope some of that helps.