Free skin?

I just ended a game, and my collections tab was lit up. I went to it. My skins tab was lit up. I went to it. I (randomly) received a Vayne skin. Date acquired is today, and I didn't have it before the last game. I told the guy I was duo'd with, and he got an Eve skin. Both of us are extremely confused about why we have a random skin unlocked. Any ideas?