Ok why does your body do it when you dont want to?

ok well today i was at a football game and this guy was cussing me out. he was calling me *****,slut,hore, and skank. anyways he did i cuz i broke up with him. but he was getting on my freaking nerves so i turned around and walked straight up 2 him and slapped him really hard... but the weird part was i was just going over there to like kinda scare himn or look at him mean and say stop, but then my hand just did it and i didnt even want it to... why the heck did it do it?