Ask your vet for something to put in the litter box so that you can get a clean urine sample. It's very absorbent and it looks like black rice. It does sound as though it could be CRF or even acute renal failure (hope not); the symptoms are there but whatever the case, I think she needs to go back to the vet, especially if it's acute b/c there's no time to lose. Please keep us posted. And keep the faythe.