What is causing it?

Right at climax I get a extreme intense headache (the one that I put my hat on).
It comes on in an instant and literally makes me think my skull is gonna explode.
This is the third time I have experienced this.

Could it be clomid/nolva?
Thats all I am taking at this time.

It is like my head is in a vice for about an hour. The scary part though is how fast it hits. It goes from "just fine" to "what the fuck!" In about two seconds.

Smart asses welcome I reckon.