I can't click on anything...

So, I don't know if this is a bug or has something to do with my personal setup. I know that it only happens in League and not frequently enough to be a grave nuisance but frequently enough to be noticeable and to have fucked me over once or three times.

Basically, for whatever reason, I cannot click on anything within the game. Both pcs and npcs are untargetable to me. I don't know how this happens and I don't know why. It just has happened randomly and the first time it happened was when I was playing a Nexus Blitz game. Really would appreciate any information on why this happens. In case this information is important: my mouse cursor doesn't use the in-game mouse cursor. At least, at first it does but whenever I move my mouse cursor to the edge of my screen it goes to the default cursor. This doesn't usually trigger the bug I am talking about but I wonder if it has something to do with that.