Britney Spears may not be the only new The X Factor judge Simon Cowell needs to keep an eye on.

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Second-season The X Factor judge Demi Lovato -- who completed a rehab program in November 2010 for "emotional and physical issues" in which she had struggled with an eating disorder and a cutting addiction -- may not be living the sober lifestyle she's letting on post-treatment.

"Demi has really busted her butt to get her career back after the incident with the Jonas Brothers backup dancer and rehab and stuff," a source told RadarOnline.

"But she's not as sober as she's leading everyone to believe and is still partying hard."

According to another RadarOnline source, the former The Disney Channel star hasn't completely tamed her wild ways although she's become more careful about hiding her personal life from cameras to try to maintain a wholesome image for her new judging role.

"She's really excited about the gig on the The X Factor and is using it as a platform to push her clean image on the public," explained the second source of the 19-year-old singer, who has many adoring teen fans.

"So she really needs to keep her partying under control."