Fullscreen Mode Not Working

For some reason today it seems that my fullscreen mode is not working. I have 2 monitors, and if I have the League game up and roll my mouse to the left, it leaves the game window and goes over to my other display. This has never happened before, so I'm not sure what triggered it. I tried messing around with all the settings in the game but nothing is fixing it. It's really annoying because I can't get to anything to the left of me quickly and end up clicking out of the game a lot. It's basically unplayable, and I don't know what to do to fix it. Any suggestions? :\