On my last run, I decided to take a chance and run all the way around the block (for lack of a better term) and then down the hill instead of going only to the top and then running back like usual. I don't know if it had something to do with me making a change, but my head started to hurt after a while. Not like it would if I'd hit it against a wall, just a sort of throbbing pain. It didn't have much effect other than to make me slow down to see if the pain would ease off.

It didn't. I had that throbbing feeling in my head for a while, and it only stopped when I started to cool down. Maybe I pushed myself too hard that day?