Benign sex headaches are severe headaches that occur during an orgasm. Despite the name of this condition, benign sex headaches are often quite painful. This condition has been known to exist since the time of Hippocrates (about 460 B.C), but only during the 1970's was it formally addressed by K. Kritz. Other names for this condition are coital cephalalgia, orgasmic cephalagia, and benign coital headache. There are two kinds of benign sex headaches, the first being described as a sudden explosive pain in the head during the point of orgasm. Patients have described the feeling to be like a sudden blow to the head, to a sudden throb like pulses similar to a migraine. The other type is described as a pain that intensifies as the onset of orgasm nears. In some instances both types of benign sex headache occur, the first type occurring immediately after the second type. There have also been documented cases where standing up after coitus has produced headaches. This type of coital headache is relatively rare. It can last for several weeks, and lying down relieves the pain.
May you never, ever have cause to read this article. Oh my god the pain. Some of the worst I've ever experienced.
I have to second that - it's happened to me too a few times. It is really, really awful.
Benign Sex Headaches - 'Coital Cephalalgia' - UCSB's SexInfo