Does she maybe need her anal glands emptied?
My cat who's now 8 had his anal glands blocked by bacteria and it burst, because it had no where to go. His but smelled bad and he was bleeding from his behind a little, where he sat. We took him to an emergency vet the next morning and they drained it and sewed up the large hole that was in the place of the gland. Maybe the anal gland needs to be cleaned out like rybett said.