Rihanna: Cancels Grammy Performance

6:16 PM PT -- A rep for Rihanna tells TMZ the singer took antibiotics for three days leading up to the show, but they didn't work. Rihanna was then examined by a doctor at her rehearsal Monday, who decided to put her on vocal rest for 48 hours because the singer was at risk of damaging her vocal cords.

Rihanna has just cancelled her Grammy performance ... TMZ has learned.

The singer arrived at the Staples Center but did not walk the red carpet. She went to rehearsals and left.

Sources connected to the production tell us, Rihanna has been struggling with bronchitis and is on meds. We're told a doctor on site recommended that she pull the plug on the performance because it could permanently damage her vocal cords.

Our sources say Rihanna wanted to perform but reluctantly decided she had to listen to her doctors. She left Staples and is going home.

She was going to sing "Kiss It Better." She even rehearsed twice but in the end the show just could not go on.