Stress, bright sunlight and barometric pressure changes. Had one for 3 days before I went to emergency room. They gave me a shot, left me in dark room for almost 2 hours. Came back and asked me how I felt. I told them the shot just 'took the edge off'. They asked if I could find a ride home. Gave me another different shot. An hour later they came back, I was still awake. They did a CAT scan checking for a brain aneurysm. Came back negative, they released me and I drove myself home. Worse pain I'd ever had in a migraine.