[CLIENT] Game keeps switching from full screen to windowed and vice-versa

Not sure if it's a bug but it only started happening yesterday. Lost 2 rank games because of this so I thought I might as well report it now rather than it happening again. So as the title say, my game keeps switching from full screen to windowed mode everytime I press a key. When on windowed mode, I can't use any of my skills or even click the options button above the mini map to try and go back to full screen manually. Only things I can do in windowed mode is move my champ (right click) and ping (left click). When I press a button like a skill for example, nothing happens until I press the enter key and it switches back to full screen. When in full screen, however, I can use my skills properly but cannot move. I can open the chatbox and type but as soon as I hit send (enter key), it goes back to windowed mode. I have no clue how to replicate this bug as it just randomly happens while I'm in game. The only fix I found for this is closing the entire client (go afk in game) and reconnect, but by the time I do reconnect my team's already far behind due to 4v5 and all I get is 5 reports. I did try to talk to them explaining my situation and said I would rather try fix my client instead of feeding unintentionally (because I can't move or use skills).

Please help as I would like to go back trying to climb the rank ladder as soon as possible.