So I just lost LP after winning a game... Can anyone help me out as to why?

So I had just won a game as Braum support.. When I got into the end game screen it said I lost -16 LP... Even though it says "Victory" right next to it.

The only thing I can think of is that I had to BRB to help my mom with something for like 5 minutes, but I came back as soon as I could. My team was cool with it because they were all so far ahead, so AFK supp wouldn't be that detrimental.. I'm not saying I'm a saint, but this DC was genuine because my mom needed help right away lol. Idk this is really odd.. My team didn't lose any LP

anyone have any insight?