I got championship Riven for one game then it went away.

Ok so I was playing ranked and I picked DragonBlade Riven and after that I went to the loading screen and I had championship Riven, I was screaming then when the game started I also had it and my whole team was like WHAT?!?! then I said I don't even have this skin whats happening? then after the game I did pretty good i went like 12/4/7 and then the skin was gone and i was hella dissapointed but I'm just wondering why I got the skin for just one match, pls answer.