It is most likely some sort of allergy but it could be caused from stress, disease, infection, fleas (if it is that bad fro fleas it is most likely an allergy). It may also be caused by some sort of discomfort, perhaps anal glands???

Has anything changed that perhaps caused excess stress around the house? Any new cleaning products?

Was she chewing the hair on her sides before & now started the same thing on her rear?

We'll be thinking of you this afternoon. I hope it's something simple, easily treatable & she's back to her beautiful self in no time. {{hugs}}