Ok, well as long as all pain is ruled out. Another thing you can do is work on round pen work with side reins. You can get a sirenge and attach side reins to it to keep her head down and pratice trot canter transitions in the round pen without you on her. This will keep her from raising her head up while engaging into the canter.

Just make sure you don't put it too tight at first, until she gets used to it.