The behavior your describing is classic for a cat that is developing bladder crystals. (theletteredcottage.net)
This is one of the only foods my cat can eat without forming bladder crystals, and one of the only foods that helps combat them. (royalcanin.com)
One needs a food with the SO factor to prevent bladder crystals, and another has a very, very hard time putting on weight due to bowel issues. (royalcanin.com)