Pain in rectum six months after anal sex sharp pain when bm?

Anal fissure possibl: If' you've had persistent pain for six months since anal sex, you need to be evaluated by a surgeon (general or colo-rectal). If you have new pain, especially with a bowel movement you probably have a fissure. Use a stool softener (not a laxative) - colace (docusate sodium) or similar - twice a day until it's easier to go. Regardless, you should see your doctor.

Tear, ulcer : You may have an anal canal tear or retal ulcer. Go see a doctor.