Guys, I've looked up on exertion headaches for a while, having suffered them quite often for the last month, nightmare!!They are caused by very strenuous activity, hence the name "exertion" headache. The blood vessels surrounding the brain are under so much pressure they expand passed natural level and push against the wall of nerves, that also surround the brain, more precisely from temple to temple all the way around. Blood vessels push squeeze against the mellinges causing the instant severe headache. Things that help; staying well hydrated (to thin the blood making flow easier), breathing correctly on positive & negative part (which is where I went wrong) and posture. Doc will recommend 2months rest, as you're very susceptible to them for upto 3 months.
Hope this helps dudes!