With such scarce information, I can only guess at your problem.
1. You don't pick your champion in time
You can pick your champion as soon as portraits appear, but if you don't pick your champion in the time allotted you will hard random a champion in the roster.
2. You're playing ARAM (All Random All Mid)
Their champions are always random, that's also the map that's just a straight line to the Nexus.
Either you joined a custom game where the game making set champion select to "Random" or you hit the instalock button. If you joined an "ARAM" proving ground map the first is the most likely option.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.