Why do i jump at loud noises?

ok so i am 15 years old and jump at loud or unexpected noises, its really weird because sometimes when i hear a loud Noise instead of jumping i just fall straight to the ground, i don't understand it. it gets so annoying like one time someone popped their chewy gum and i jumped and another time i was just standing up talking to my friend and my teacher screamed at someone across the room and i just fell to the ground. something that might have something to do with it is that when i was 10 i fell through a roof and hit my head really hard along with many other injuries. but this jumping and falling thing only set in when i was about 12. please help me im sick of just walking around the school yard and then the bell goes and i fall to the ground. its very embarring. so does anyone no why this would happen and what i could do to fix it