Stage fright, or any kind of stress, changes our breathing.
The body breathes in two different ways. We breathe deeply, all the way down to the pelvis, when we're asleep and when we're not stressed. But when we're stressed or anxious, we breathe shallowly, with increased blood pressure and heart rate, sweaty palms, dry mouth–begin to sound familiar? Like stage fright?
Stress breathing happens automatically when we're anxious. But it's an autonomic function–that is, one that we can change if we want to. We can learn to override the function and reduce nervous system activity.