UPDATE 3:06PM: According to a source close to the situation, the earlier report from TMZ is not true and Rihanna pulled out of the show due to bronchitis.
See below for the original story.
During the Grammys, news broke that Rihanna canceled her performance earlier in the day because she had bronchitis and wasn't cleared by doctors to sing. Now there are some conflicting reports as to why she pulled out of the show, and according to TMZ, she freaked out during her final rehearsal earlier in the day. It's unclear why Rihanna was reportedly upset, but the report states that she was screaming hysterically over the rehearsal and eventually stormed out of the venue.
She was reportedly so upset at something that she didn't even want to stay for the show. To further cloud the situation, video surfaced of Rihanna singing flawlessly during a Lionel Richie tribute just two days before the Grammys, and sources told TMZ that she sounded great during rehearsals. Rihanna has yet to comment further on the situation, but did apologize to her fans on Twitter for not making the performance, though she didn't go into any details. We'll continue to update this story as more details come out.