Hmmmm. Sorry, hmmm is all I can come up with. The obvious is to search back in your mind for what might have changed to prompt this. And to Vet check.
I've only ever had one cat spray in the house and that was our Sadie-cat, who only did it when we first got her. She was pretty beat up when we live trapped her off the street and supposedly pregnant (though she never had kittens) and she continued to spray for about the first year we had her, even after spaying. We had to throw out some pieces of new luggage that she got inside and sprayed in. Then she just never did anymore. I mean not inside, she continued to spray outside, backing up to flowers and bushes instead of digging a hole. She did dig holes in the kitty litter inside.