From usmagazine.com:

Nicole Richie Enters Treatment Program

Nicole Richie has entered into a
"diagnostic treatment program" to determine why she hasn't been gaining weight.

"She's not being treated for an eating disorder," a Richie source tells
Us Weekly. "They're trying to figure out why she's not gaining weight."

Another source tells Us that Richie may be suffering from a rare blood
disease, and that tests may reveal a connection to her inability to gain weight.

In the meantime, shooting of The Simple Life 5, which was scheduled to
start next week, will be delayed by at least one week.

Richie's rep,
Nicole Perna tells E!, "It is important to Nicole that she achieves this goal in
a healthy way as this is not a treatment for an eating disorder."