Suggest Remedy For Pimple On The Right Buttock

Hey, so I have a rather unpleasant one for you. Just outside the hole of my anus, on the right cheek, there is something of a pimple that once perhaps had a head but persists now without one. The alarming thing is that most of the time it recedes to a size well beyond negligibly miniscule but every three weeks or so expands to a bulbous size, fat with blood and even inhibiting my ability to defecate by simply being so large that it blocks passage. This will only persist for a few hours and the growth will bleed out surprisingly little before contracting to a size smaller than a pimple or bug bite. Unfortunately, this has gone on for over a year now. I am 25, 6 feet in height and I weigh 195 or so. Also, I am not on any medication, but I have undergone light surgery to remove sebaceous cysts on my scrotum. How concerned should I be?