While Nat is practicing onstage, what happens for the third time that day?

From the text:

As I stood there on the stage, for the third time that day there was the weird blurring around me, as if I were underwater, and a buzzing in my head like the voices of a crowd, and through it a faint thread of music. The stage pillars and the galleries beyond them seemed to tilt and sway, and I felt myself stagger.