[GAMEPLAY] strange behaviour in game

when i start the game everything is normal in the beginning after few minutes the game starts lagging another few minutes everything in the game acts strangely my champion responds when i move it but it doesn't attack sometimes it also return to the base on it's own and then appears again where it was without me doing anything other ally and enemy champions/minions are freezing this happened twice in co-op vs AI summoner's rift game and i had to leave the game