I had a kitty who had that problem. Her name was Sassy. It turned out to be a kidney disease. Sassafrass(Sassy's real name) was very lucky that we took her the vet when we did, because if we had waited any longer, she probably would have been too sick to treat. Sassy lived for 5 years after we found out, geting fluids once a week, and near the end, twice a week, but this fall, she stopped eating and had withdrawn herself from people. We had to put her to sleep, and were very lucky that she lived those 5 years, because the doctor said that she would probably only live for 2-3 years. She was 13.