After a year of this I noticed a big painful lump in my perianal area.... (ashotofadrenaline.net)
The cause of anal furunculosis is thought to be a combination of the animal's immune system overreacting to digested food and / or bacteria in the perianal area, and faeces getting stuck in the perianal area. (petsmagazine.com.sg)
Another thing that may well help is to make sure the perianal area is kept clean with baby wipes. (petsmagazine.com.sg)