You are suffering from a condition called Anal Fissure. Sometimes, while passing hard stools, a tear or split occurs on the edge of the anus. There may or may nor be any bleeding or pain the first time it occurs. The tear heals by forming a scar. But soon, when another bout of constipation occurs, the tear occurs again at the same place, breaking down the scar. The cycle is repeated - a scar is formed, breaks and then occurs again. And each time, the scar becomes wider and deeper, causing narrowing of the anus. In tine, even passing of normal stool or even soft stool can be painful, since the anal canal is narrowed down and cannot dilate.
The treatment for the condition depends on how far advanced it is. If diagnosed early, stool softeners, antibiotics and anti-inflammatory medicines and lubricants to apply around the anus can often be enough. If the condition has been occurring for a long time and scarring is extensive, you may need surgery to cut away the scar tissue and enlarge the opening of the anus. Regular dilatation of the anus is also necessary. Plese see a proctologist (a surgeon who specializes in diseases of the anus and rectum) for treatment.
I hope this helps.
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