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If this is on the top of her tail a few inches up from the base it may be she has an infection in the sebaceous gland there also called the supracaudal gland. In most dogs that isn't active but in some it gets clogged up and can get infected.
Another possibility is that this is from flea bite allergies. The area around the tailis frequently where the dog reacts by being really itchy and a dog may groom hair off tail and rear when trying to resolve the itching.
If you see a lump along the top of the tail then you could try cleaning the spot with 3% hydrogen peroxied and applying some Neosporin or Triple antibiotic ointment to the spot.
If your dog is not on flea prevention then using Frontline may resolve the issue.
If you are not sure this might be another problem entirely such as mange mites, ring worm fungal infection, skin infection, an irritant the dog sat in causing the problem etc.
I'd plan a vet visit to check out the area and figure out how best to treat the problem.
Hope this helps you!