Actually, she didn't win the contest. She made it to the final 12 on American Idol and was told by Simon Cowell she didn't have what it takes before she was booted off. Which I believe was about 2 1/2 years ago.
She competed with about 700 other girls for the part in Dreamgirls.
When you take into consideration the casting, rehearsal and movie making process, she's been busy. Not to mention all the talk shows she's been on.