Why is My Batman Arkham Alylum Switching to Joker character and starting over, when I head to killer crock?

Why is My Batman Arkham Alylum Switching from Batman to Joker character and going back to the start, when Im headed to killer crock in the game? I did download the Joker pack, but I umed that was its own thing, not that it would switch over. How can I fix this without loosing my game data?