League keeps alt tabbed all the time

Hey, for the past 3 days I cant play LoL. When Im in lobby, its fine but when I start loading into the game LoL just alt tabs and when I try to get back in the game It just alt tabs again and I already lost 5 games cuz of It. I reinstalled LoL 5 times in a row and it worked fine only at the 5th try. Yesterday, after 3rd game LoL started microlagging so I stoped playing. Today, It is alt tabbed all the time again. Please someone help cuz I dont know what to do anymore.