Is your cat indoors or outdoors?
Does she use flea prevention?
The two possibilities here are either a tapeworm or an anal gland problem. The most common would be a tapeworm. You can deworm her for this by giving her an over the counter worm medication with the active ingredient of praziquantel. If she doesn't quit scooting in a couple days then you will need to visit your veterinarian to have her anal glands checked to see if they are impacted. If they are your veterinarian can empty these glands out during the office visit.