Why do I freak out so bad ?

My brother knows it bothers me he points a knife at. I'll start to freak out badly to the point where I would have a breakdown almost. My body just freak out and takes over and I go into run away mode. It even makes me uneasy if a knife just laying on the table no one near it and it's pointing at me ill swat the point to where it's not facing me. They one day my brother pretend to stab me with the back end of the knife. My body froze and when he did I started to breathe shallow, like I was excepting to be bleeding. Are that anything reasons why I do this ?