Dog Had Puss Pocket On Belly. Anal glands dogs also commonly suffer from anal gland abscesses, in which the area around the rectum becomes red, swollen, and tender. These can be caused by a bacteria infection of the hair follicles (bacterial folliculitis), autoimmune diseases, a bacterial.

If the swelling was not caused by the accumulation of puss then it could be due to a hematoma (the buildup of blood). This area can fill with abnormal fluid and cause problems for the dog. Rough play or fighting can also cause the trauma that results in a hematoma.