[joy and bella]

so bella has been draggign her butt....i thought it might be her anal glands..and than i thought worms...i had her anal glands done when i got her groomed (or at least they said they did them) and i took a sample stool to the vet. thevet said teh stool came up negative...but she is still dragging her butt??? and now she is biting at it. they said to lookfor tape worm ...but i dont see any??? should i take another sample???