I also got an exersion headache doing 20 rep squats, as others have posted:
my head felt like it was going to pop
after resting it backed off, but as soon as I tried anything bang it was back again. was like an instant migrane, i had light aversion, and dizzyness aswell
my ct also showed no issues
massage didnt help a hell of a lot
took me 2 weeks before i could relly push myself again
My doc said it was probably due to the bar sitting on a nerve or vein that drains blood from your head, your heart pumps like a bitch and forces teh blood up there, but because of the bar position, or overly tensed muscles it reduces the bodies ability to drain blood from the head, My doc said it had something to do with the "circle of willis" basically a common blood vessel that links most of the ins and outs
Rest was the only thing that helped me in the end..