I won my game and LOST LP. It said this was due to AFK behaviours. I never went AFK.

I just won a ranked game and lost LP. I never went AFK, never even sat in fountain for more than a few seconds to buy items. I was active the ENTIRE game. I didn't even D/C for a moment. Why did I lose LP for this? That is wholly unfair. I've never went AFK or RQ.