Shih Tzu with itchy bum?

Last November, my 3 y/o started scooting on the carpets so I've been taking her to the groomer every week to excrete her glands. On Monday I took her fecal sample to the vets and they came back negative for any parasites or worms. I'm taking her to the vet for an exam on Wednesday. Anyone know why this would start suddenly and/or what the vet may recommend?