Sounds like it's rider error. Ride her the area or round pen and have someone video you. Look at your seat, hips, shoulders, and head to see what you're doing. My mare did something similar for a while and I figured out I was dropping my right shoulder just a little and didn't realize it. As soon as I worked on keeping my shoulders straight, she went fine!
Also, Mustangs usually mature a bit more slowly than most QHs (or at least slower than people think QHs mature). If she's comeing 4 you're fine, but if she's just turning 3 this spring, then you might think about taking it easy riding her until last next fall.
"To be an equestrian in the classical sense is not just to be a rider. It is a position in life." --Charles de Kunffy
Anti-Slaughter and Proud!