Sounds like a reoccurring anal gland abscess that can be drained. Do not wait for it to rupture. Various infections cause this.
Usually benign, tumors of the sebaceous and modified sebaceous glands are quite common in dogs. They include nodular hyperplasia, sebaceous adenoma, sebaceous ductal adenoma, sebaceous epithelioma, meibomian adenoma, meibomian ductal adenoma, meibomian epithelioma, hepatoid gland adenoma and hepatoid gland epithelioma.
Malignant sebaceous gland tumors are sebaceous carcinoma, meibomian carcinoma and hepatoid gland carcinoma.
The vet must refer you to someone who can do a proper diagnosis, otherwise, he/she is just guessing at a solution, which is horrifying.
