I lost LP in a ranked game even thought i won the game

So I was Nasus in my ranked game and later in the game I had a blackout which I couldn't control. The Blackout was only like 5 mins or so but I did come back and my team won the game. After that, I went to end screen and I see on the op that I lost 19 lp even though I won the game. It is because of I afked or was it something else? Please help me I don't understand what happened.