Why is my screen acting weird

Why is my screen acting weird

So when I want to do the thing where you do a number then enter then add/subtract/multiply/divide then enter multiple times so it goes up by increments (don't actually know what it's called) I can only press enter 5 times and then the screen goes wonky if I try to enter more than that. I don't know the technical terms so this is probly hard to figure out but some help would be great ^~^ thanks