11/20/2007 AT 12:05 AM EST
has been hospitalized with a kidney infection, her rep tells PEOPLE.
The actress entered the emergency room of a New York hospital on Monday.
"Mary Kate got a kidney infection," says rep Nicole Caruso. "She's resting comfortably and will be released in the next day or so."
The actress, who turned 21
in June, recently said she was in good health
"Mentally, physically, I feel pretty on top of my game right now," she told EW.com in September, as she prepared for her debut in Showtime's Weeds.
"Talk to me next week, I don't know. Today, I feel good."