Mystery Damage, No source

Death recap shows "290 damage" from blank source. Yes, I took damage from Gnar, but this is the third game this has happened. Last game was a dead kha zix that was dead and not even in the fight. I take mystery damage, oddly, enough just to kill me while the champion is not currently engaged with me. This happens when you're walking away from a fight that you'll survive, then suddenly, you'll just die with the exact amount of damage it will take to kill you.