The most common causes of anal pain after bowel movements are anal fissures (caused by a crack in the sensitive skin lining the anus) and hemorrhoids. Most of the time, both of these benign anal conditions will respond to conservative measures, including increased water intake, increased fiber in the diet, stool softeners, and avoiding straining and prolonged sitting on the toilet.
There can be other causes of anal pain as well, including, rarely, malignant tumors.
If your symptoms persist despite increasing your fluid and fiber intake, then you should ask your physician to evaluate you further.
Sincerely/ Robert A. Wascher, MD, FACS
http://doctorwascher.com