As far as I know, this is a known but that occurs when you buy a champion and don't close the game after doing so.
That's one of 3 things that could have happened.
The other two being that this is a bug that is actually persisting even when you've closed the game and reopened it - therefore definitely needing to be reported to technical support.... OR, now I'm not calling you ignorant but it happens to people sometimes, you bought a character that happens to be free this week and just haven't noticed it yet.... jsut sayin', **** happens ya know? 