Obviously make sure you keep her litter tray in the same place, somewhere quiet. Clean up accidents with warm water (disinfectant & similiar products are scented and attract the cat back to the spot) and then apply surigical spirit or a special urine deorderiser.
My cat started getting cystitis and inbetween the second and third occasions there was a period of two weeks. The vet had initially treated with antibiotics on the first three occasions, but suggested it was worth having an xray and urine analysis done. Therefore, I think it would be worth asking the vet if any tests are worth doing as these can rule out irritation by crystals or other, or infection (and the vet can treat accordingly). The tests were inconclusive for my cat and the vet came to the conclusion it was cystitis and prescribed Cystease (Cystaid is very similar) which is great and can be mixed in with wet food to avoid pill pushing. It really helps. If the tests reveal don't reveal anything, then I would guess it's cystitis or behavioural. If the vet thinks it's cystitis, ask about Cystease/Cystaid. Obviously if this helps you've solved the problem. If not, then it could well be behavioural and you can get further advice from the vet and others on here.
Your cat may not have cystitis, but I know it's very stressful to have them weeing around the house and to keep cleaning. Let us know how you get on her with.