Exertion Headaches...? Anyone?

I am fairly certain i have been experiencing exertion headaches for the last few workouts ive tried.
i have always been terrible at drinking enough water, and over the last few weeks ive been lifting a lot more weights and lifting heavier every week.

the first one, started on a Wednesday morning after a heavy lifting day... i got up to swim laps at the gym like i always do on Wednesday mornings, half way through my workout, a horrible headache had a vice grip on my head (left hand side of my head, at under the eye brow near the pressure point, and at the base of the skull on the same side) . i managed to finish the workout, but by that time it was so bad it made me vomit in the shower.

Thursday, i got 2 tracks into my les mills body pump class (weight lifting class at the gym), half way through the squat track, the headache started creeping back.. it stayed with me through the workout, i had to slow down and lift lighter, but i still finished the workout.

Friday, again in the pool... was fine when i started, but after 4 laps the headache hit so hard it was making me nauseated and i had to stop... only got 24 laps in, i usually swim 60.

Sunday, tried to do a light workout with my bf... did 15 min on the elliptical, no problem... went to grab a 45 pound bar and do some squats... 6-7 squats in and it felt like a bomb went off in my head.. still left side. had to stop after one more set of squats and a set of dead lifts... it hurt too much to continue and i didnt want to hurt myself since i wasnt sure what was causing it.
the headache eased up after a couple hours, but has lingered even till this morning, my head is tender and my eyes are sensitive to light, similar to migraine pain.

im eating plenty, sometimes too much, and i get enough protein.
i know i need to up my water intake... but what else should i do do prevent this from happening anymore?

(side note: i had started using Thermovex last week also... i only ever take it in half scoops, and i took c4 in half scoops for months before this with no ill effect.. so i know its not the stims, to be safe however... because i wasnt sure it WASNT the thermovex, i stopped taking it, last thursday was my last dose and i still got crippling headaches friday and sunday... which makes me believe it IS Exertion Headache from lack of hydration .

anyone have any advice or personal experience?