What Mrs P said.
Milly went through a stage of weeing on her radiator hammock or the bath mat. She had cystitis because she's highly strung. She doesn't like out doors either. The vet suggested to me:
1) make sure she has a clear view of outside from the door. If it's other cats that she's scared of, she will be able to see if they are around and if she's safe to go out.
2) have more than one tray, cats are fussy.
3) keep trays out of the rooms where she eats or sleeps - did I mention cats are fussy.
4) put the trays in places where she won't be disturbed when using them.
Milly has a tray in the bathroom - to stop her weeing on the bath mat, one in the utility and one upstairs. She uses the utility litter tray for poos and wees in the other two.
The vet also suggested feliway, but I'm not sure that helped her. She also gave her some herbal tablets - some mother cat milk product that calms cats. This helped her calm down and the weeing issue stopped.
I hope you pus is ok.