Lump outside the anal area and pain.

Hello Doctor! I have a lump or a small sack hanging outside (slightly closer to the left side) the anus. Till the date it was pretty much normal. I had experienced some minor pain and itchiness during and after bowel movements in the past but they didn't last more than two days. But almost a month ago I started to feel a slightly sharp pain inside (left side) the anal area. Then after few days I started to feel a sharp disturbing pain sometimes in the left side, sometimes right side and sometimes both (total anal area). And the small sack like structure swells when there is this sharp pain. Do I need to be worrried? Is it treatable? I'm afraid to go to the doctor so please tell me what I have and please suggest some medicines. If I must visit a doctor then who should I visit? I currently live in Kolkata. Thank you.