The musician - who won the first series of the TV talent show in 2002 - did not consider herself worth of being a pop star and thought she would have a career as a back-up singer.
She said: "My goal was just to be a back-up singer. I never intended to be in front.
"But then my apartment in Los Angeles burned down and I had to move home, I had no money and I had to sleep in my car for three days. I just auditioned for this thing that said they'd pay you, and it happened to be 'American Idol'. I didn't go into it thinking this would happen, I went into it thinking it might pay my electricity bill."
She continues to be shocked by her level of fame and does not know how she did so well during the series and in her subsequent career.
She said: "Literally my career is happenstance. I work very hard, but I always wonder how in the world I got here."