Cats dont tell you when something is wrong, so any change in behaviour, especially around their bathroom habits, is reason for a checkup at the vet.

I had a cat start peeing on the toilet, and have heard similar with cats starting to go in the shower drain, and turned out to have pretty severe kidney damage.

Hopefully it is nothing like that, but I didn't realise something was wrong until it was too late and I don't want the same for you and your cat.