Perhaps she is anticipating and stiffening with the anticipation. If she gets hard in the face and her head pops up, she is stiff or stuck out somewhere behind the head, the head is where it shows up at. It may be a physical issue or maybe you are asking too abruptly?
As far as the lead changes, it may help to just trot and lope around the arena and practice moving the hip and the shoulders where ever you want. Do not worry about changes or circles. Make sure you can position her and she responds.
Later you can go back to loping circles, setting her up for the change but counter arc on the same circle, or counter canter. If she rushes through the middle, really drive her over the bridle and push that hip out. It is a lot of work to rush counter cantering. When she softens, ease up.
Change it up, keep her guessing and do not let her change until you tell her to and make she is so soft. Don't make the center of the arena always about changing leads. If you are counter cantering and she is soft you can ask at any point on that circle because she is set up for it. You can counter canter loops and change on the long side of the arena. You won't feel rushed to change before the wall. Or ride out in a pasture.
Make sure you are relaxed yourself and not trying to throw her into the change with your weight, looking at the new lead shoulder or suddenly turning in the direction of the change.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.