I am sure that nausea after sex is sometimes related to oxytocin or other chemicals released, maybe estrone, cortisol, prolactin, oxytocin or serotonin . My wife often feels nausea after sex and also right after letting down when breast feeding. She reports that it feels very similar and she thinks in her case it is related. I have read that these chemicals are released internally during sex and are also contained in semen. I wish we could find more info on how to prevent the nausea. Other possible causes that I've found mentioned but don't seem to fit our situation are lack of oxygen or holding breath (valsalva maneuver), intensity of abdominal exercise, hitting the cervix.