severe rectal pain after sex in male

for the past four years or so, randomly, sometimes I get severe, buckled over, rectal pain, deep inside.... feels like a knife being plunged deep inside me after sex. It can take up to an hour or more to settle back down..... feels like a major thing....dying in pain.....sometimes right after an orgasm, sometimes the next day.... but it is crippling, can do nothing but wait it out to go away..... ... feels like a prostate spasm of a sort, but doc says prostate is normal. Doctors have no idea what it is