I had a similar problem. There was an overactive gland under my skin near my anus and it was finding ways of getting that nastiness out to the surface. See your doctor in case you have the same thing...

Does it hurt? If so I recommend taking a 20 + minute sitz bath in very warm water and perhaps clean it then too. It may need to be drained by a proctologist if the bath doesn't relieve the pain. That can be VERY painful.