Sounds like fiber and water will help her. You could try some beans or some canned pumpkin to get some fiber in her. Ice cubes, pedialyte, letting her lick the water off your hands-do anything to get more water into her. Moistening her food with water might help too.
Call your vet in the morning-they may want to see her, adjust her diet, or prescribe some ointment to help her little butt heal.
