Nicole Richie has checked herself into a "diagnostic treatment program" in a bid to find out why she can't put on any weight, according to new reports. A source tells Us Weekly magazine that the skinny star fears she may be suffering from a rare blood disease, and that tests may reveal a connection to her inability to gain weight.

The source says, "She's not being treated for an eating disorder. They're trying to figure out why she's not gaining weight."

Richie's publicist Nicole Perna tells the publication, "It is important to Nicole that she achieves this goal (to gain weight) in a healthy way."

Richie made the decision to check into the treatment center after spending months with a nutritionist and a therapist, trying to get to grips with her weight issue.

(This news article provided by World Entertainment News Network)