Howcome every time i see my Ex gf it pisses me offlike almost loose controll pisses me off!!!!?

i had been doin good and had almonst been fully healed from my breakup with my gf we had dated for about a year and accomplished so much and one day she just got up and walkedout of my life and i was sooooooo sad but i have healed alot since then. but today i went out for the day with all my guys friends about 8 of them to be exact and one of then shows up from around the corner with my Ex and i just started to blush andg et very very mad! if she is just with in 60 feet of me i get very mad. why??