Red border during game?

So I was playing a game in co-ops, but there was a red border that appeared around my game at about 2 minutes in. I couldn't move my mouse outside of the border, so I ended up using locked camera screen, which had the most convenience during that time. This was the 2nd time that this happened, except it happened late on the 1st game. I tried narrowing the screen resolution, which showed a red border around only my game, not my laptop screen. It's distracting and inconvenient for my game play. I would appreciate any help that I can do to fix this. If this is a bug, no biggie. Thanks! =D