Died tought i won the Round

I just lost a round even though i beat my opponent. I was on 5 HP and the opponent left before the game was over. I think overtime just started though im no sure. As soon as i killed his last unit (3 star garen with frozen heart, trap claw and something else), i took dmg and was out of the game. Also when i look at my match history it shows way different champions then i was using. probably unrealated but i just saw that aswell.