"Nicole has checked into Cedars-Sinai where she is being treated for pneumonia," a rep for Nicole said in a statement to the online outlets. "She is doing well."
Over the weekend, Nicole, 28, posted a message on her Twitter page noting that she was feeling under the weather.
"ok cold, it's been 6 days. Lets part ways graciously & keep it moving," she wrote.
Earlier this week, Nicole was in a Los Angeles courtroom where a judge granted her and longtime beau Joel Madden a temporary restraining order barring two celebrity photographers from coming close to her or her children.
Nicolegave birth to son Sparrow James Midnight Madden in September. She and Joel also have a daughter, Harlow, who turns 2 in January.
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