Oh, come on guys. She's preformed this role hundreds of times, she's not nervous. The look you all saw in her eyes was something called acting. At that point in the show she's just had her world shattered and she's trying to decide what she's going to do.
Her breathing ALSO comes out of this thought. Her CHARACTER is very unsure and nervous about what she's doing. If y'all go back and listen to RENT you'll see that the breathing thing is not a personal trait - just her character's trait.
Sad she couldn't hold that last note, tho.

Doing that 8 times a week isn't easy.