Dark coloured stool, lump in anus.

I have a small lump hanging outside the anus. It's a sack like structure. It didn't cause any severe pain before. 3-4weeks ago sharp pain started during bowel movements and the lump swolled up. Now every 5-6 days gaps I get this sharp pain. When I take sitz bath with hot water I feel better. Sometimes I feel the pain only in one side of the anus and sometimes the whole area has a sharp pain. I cannot sleep at some nights. There are days when I have problem sitting for a long timme. The stool coulor is dark. The pain had stopped for 3-4 days after taking applying hot water in the area but now again I feel pain in the anus and the small lump/ sack in swollen and it hurts too! The anal area is itchy too. Please suggest some medicine and remedies. Is this a hemorrhoid? Do I need to visit a doctor?