This is a sign that your cats glands are not draining naturally. Take her into somewhere they groom cats/dogs they will know how to do this and it wont cost much. If that is the case and you don't get it taken care of it can cause infection.
Why Does My Cat Drag His Butt On The Floor?
We have a male cat who drags his butt across the grass in the back yard. What might his problem be?