The movements of the body at a canter are different, for rider and horse, and many times that is where bucking issues will come out. Saddle that is "okay" at lower gaits, will not be find at canter.
Could be there is a deep seated abscess in muscle tissue, could be thorn has worked its way into area where girth will lie, could be she fell and broke something too, cracked ribs, spinous process, withers?
Could be she has cracked a tooth, or has something wrong in mouth.
Your saddle could also have internal damage, nails, screws, slivers of wood? Could be also tree is broken and causing pain.
Might be that you are not a steady rider, and have bad hands and she is finally fed up with it? Your curb/bit/bridle may not be fitting right.
You can discipline her all you want to, but if the issue is pain related, that is neither productive nor is it fair.
Something has changed, and only by getting horse to equine vet is going to tell you if it is pain or not.
A video of you riding her would help.
Horses make me a better person.