What Is The Painful Knot On The Anus?

When i woke up this morning my anus was feeling very uncomfortable and kind of hurting. Its been aggravating me all day. I looked in the mirror but dont see anything there but I can feel what seems to be a small knot under the skin in the left side of the anus and it hurts when pressure is applied and when my 'cheeks' are spread and no bleeding. I have not been constipated or straining as my bowels are always diarrhea like since i had my gall bladder removed. What could this be? And will it go away on its on or should i see a doctor?