What is the main conflict in Airhead #2: Being Nikki by Meg Cabot?

Airhead #2: Being Nikki is the story of Em, an average girl whose brain has been transplanted into a supermodel's body. She learns how to navigate the duties of "being Nikki" such as modeling, juggling school, work, and boys. In addition, Em also has to figure out exactly why her brain was used to replace Nikki's and what "crime" that Nikki committed against the corporation she worked for, Stark.