Ok! When a cat stops using the litter box, many reasons can cause it. Since yours is female, I have to remind you females can and sometimes DO spray.
Back on topic. UTI is a top reason for stop using the litterbox, she may be having trouble 'going' or can't stop herself when she does. Have you noticed any yellow spots on the floor, couch etc? Or have you noticed any traces of blood? I'd take her in for a vet check for a UTI. It could also be an unknown issue inside her little body, like Kidney failure. But I think it's a UTI that your having, or she just learned to not like your litter!