Slave to the Hairy Hikers
Exactly where is the lump? Right at the base of the tail, or just below, to the left or right of the anus? If it is next to the anus, I'd go to the vet straight away, as it will probably be a blocked anal gland that needs squeezing out. The longer you wait, the more painful it will get, and if it bursts, she will need antibiotics.