In a post on Twitter.com, she tells her followers: "Bronchitis and laryngitis boo! I'm still goin to (Grammy) rehearsals tho! Sunday is everything."
The Umbrella hitmaker was due to perform five tracks at the Unforgettable Evening gala on Thursday (10Feb11), but she pulled out at the last minute to rest. She was replaced by Cee Lo Green at the Women's Cancer Research Fund event in Los Angeles.
Rihanna was previously forced to axe her slot at the 2009 Grammys after she was attacked by ex-boyfriend Chris Brown at a pre-prizegiving party.
Rihanna's single Only Girl (in the World) has been nominated for Best Dance Recording, while Love The Way You Lie, her collaboration with Eminem, is up for three Grammys at this year's (11) ceremony.