why is my screen freezing up with flickering lines at top?

i've had my iphone 6 plus since february and its been doing just fine. recently while in class my phone dropped but not too harsh. while i was in an app texting the top of my screen had white blank and striped lines flickering back and forth and while that was happening my screen stopped working, i couldnt scroll or swipe not even go into an app. when i restart it ...its fine ...but then in a few minutes it starts again and then for a second it's working , never has this happened and im really scared because i dont think i can get another one. some one PLEASE HELP ME!!!