Thanks for your query.
You have apparently a perineal abscess
(a large boil near the anus); the infected material has liquefied into pus. The abscess has burst, releasing the pus.
In the background of chronic constipation
, friction by hard stools cause bruises in the delicate anal lining, through which infection penetrates into the surrounding tissues (inner part of butt cheek) or even form a fistula communicating between the anus and the abscess.
Consult your doctor and apprise him of my opinion. I am certain that he will agree with me. He will examine you to rule out fistula formation; put you on antibiotics and anti-inflammatory drugs; if required, he will give a small nick to completely drain the abscess.
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Dr. Rakesh Karanwal