Image size changes constantly ...

I have a Toshiba Satellite C55-A . Ever since Ive bought it, while I am using it, typing, reading whatever it is Im doing it constantly changes in size. It will go up to 150% down to like 50% and I dont understand what is making it do that. Every time the screen changes in size Ill get a pop up at top of screen asking if I want to restore default and that will make it go back to 100%. What am I doing to make that happen? And how do I stop it???? So frustrating!!!