this kinda stuff happens to me all the time. i think its because when you flex your muscles and stuff you put alot of pressure on your head and face when you clench your teeth and stuff and your blood begins to build pressure up there. Im sure you've done it before on purpose to, have you ever tried to make your face red just for fun? yeah, if you didn't already know though lol. but you coulda popped a vessel maybe, if it still hurts, go to doctor.