Sounds exactly like Ivy's symptoms a few weeks ago. She had a urinary tract infection. A week of anti-biotics cleared it right up. You might want to ask your vet(assuming that is the problem) about a couple days worth of anti-inflammatories too. Ivy took Metacam for 3 days. Her urinary tract was inflammed so it hurt to pee, even when she did have a full bladder. The Metacam made her much more comfy and now she doesn't associate the litter box with pain.

Hope Abby will be all healthy again soon.