How long has this been going on? Does she have any sneezing, runny nose, or watery eyes? Somechickens can get a temporary high pitched stridor when a small piece of food gets in the airway, and most of the time it clears in a few hours. If this is not the case, she could have a respiratory disease or something else affecting her airway.
I just realized it today when I went to grab her. She hasn't had any of those. At least not that I've noticed. I didn't really get to see her that long but the only thing I recognized was the harsh breathing. I will check tomorrow. Hopefully it's just food.