Hmm...well, looking at the way she's dancing, it seems like it's not something physical like twisting an ankle. Coming from someone who's done that countless times, she would be carrying herself differently and her footwork would probably be off. D: That's good, at least. Maybe she started to feel sick or something like that. D: Either way, man, it had to have really effected her to make her not smile the entire performance.
It's really hard to not smile with that song.