Fps drops

Hi plz i have this issue that i never had before my fps drop when i leave lol the moment i die i go check on fb and stuff but when i return fpa drops to 30 and 40 i have a asus rog this never happen to me before i have good cpu i7 and a gtx960 4gigs and 32ram this problem only in league could someone help and the pc power option is in high performance and thnx