Hi - Iam new to the forum also but had a sort of similar problem. Myhorse, who i had 3 years, suddenly just blew up and i went off. no fun (and 30 stitches on my face where i caught a hoof).
I just had a trainer/body worker go over my horse and she determined a)dental work needed, b) feet still bad (although has had much work done) c) the bad feet caused sore withers, and d) the saddle just didn't fit well. The combo added up to, "you gotta go!"
I totally agree with Frog, i.e., check the blanket, make sure the saddle fits. My guy was "vet checked" also and three weeks later i was soaking his little feet in magna paste and duct taping them. Vets don't always get everything. You want to rule out any physical thing before you assume it s a behavioral thing. A friend of mine just had her horse "bodyworked" and it returned her sweet horse to her (he had beena real pain)
I would ask the previous owners exactly what her life was like when she was "perfect" - how often and on what was she turned out? what was she fed? how much was she worked? does she buck when you lunge her?
Hey if she keeps bucking like that maybe you can take her to the Miles City Montana Bucking Horse sale (just this past weekend but they have them every year) maybe you have a talented bucker on your hands!