Head going up = hollowed out back = pressure on the hocks.
Few issues to check out. Teeth, poll, back and hocks.
Teeth - bit or teeth themselves
Poll - alignment or how does the bridle fit? Gap when you apply pressure or does it lay flat?
Back - saddle - etc
Hocks - sore? Does she roll back nicely over them when you ask her to?
All of this is good advice and should definitely be looked into, but you also will want to consider that a horse who was trained to spur stop wouldn't be used to the rein preasure and would potentially throw it's head as well.