Well, you know medicine isn't my forte, but perhaps it's a matter of blood pressure? The "Big O" is at least on my top five on things that'll quickly spike a heart rate.

I google'd your question after typing the above, and I got the link at the bottom of this post. Your disposition to migraines (according to the article) puts you in position for them, and my hunch was correct according to the article, or is at least theorized as correct. I didn't look through the entire thing, but perhaps it can be of some use?

Hydration...I don't know the correct amount, honestly. I do think it could be a factor though. I "smoke" electronic cigarettes after quitting the real thing, and the medium that's used as a base in the juice causes dehydration after inhaling for a good bit (and I inhale a LOT, so... ). I get headaches if I go too long after "smoking" without drinking something (have one now, actually ). For me personally, a bottle of water does the trick, but that varies. Why not try starting there and varying the water intake as you need to help with hydration?

Orgasm and headaches