Why did my screen blow up to twice it's normal size?

So I was using my Iphone for a few minutes texting and then the next time I went to unlock it the screen was over twice it's normal size. I can only see a few icons and to see the rest I have to put my finger on the side of the screen and it then will scroll over. Anyone know what happened or how I can fix this? I have turned it off and on many times but that didn't work. I synced it with itunes but nothing. This is very strange.