Our rescue male Westie had a almost funny was to scratch his anal (area) itch. He would sit on the rough finish carpet and spin around, not drag. I too though it was a sac problem, but was not.
His annual physical came to his aid. The vet raised his tail (the Westie is characterized by holding its short tail in a vertical) and pointed to a red rash area just at the base of his tail. He had some other problems and is still taking antihistamine, antibiotic, and an internal yeast fighting medication. I also cleaned the area at least once a day with a "hot spot" over the counter medication. He seems to be okay now, no more sit and spin.
Where is it itching? Under the tail in the canal or vulva area or on top at the base of the tail?
Has she been wormed? Dogs with worms and itchy bums will show this behaviour. Just guessing ...