When you say buttocks, do you mean near her anus, or some other area on her rear? There are several possibilities that come to mind, but it depends on location.
If it's near her anus, it's probably an anal sac. There are some other lumps that can show up in that area, but the most common is an inflamed anal sac. She'll need to see her vet.
If it's on some other area of her rump, there are still several possibilities. A histiocytoma is a benign skin tumor that is round, red, and usually not painful. They're not all that uncommon, either. If the lump is dark red or reddish purple, it could be an hemangioma. This is a relatively benign skin tumor.
Dogs rarely develop boils. But if they do, they're usually very painful. Boils are a deep skin infection, and need to be treated with antibiotics.
In any case, a lump should always be evaluated by a vet. Most skin lumps in dogs are benign, but there are some that are malignant. You want to be sure what you're dealing with, and take the proper course of treatment, even if the proper course of treatment is to do nothing.
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