I occasionally have this problem with my boy. It only happens once every couple of months, but it always goes the same. Usually on those days I'll get on and walk/trot, and when it comes time to canter he'll pick up this slow cruddy canter. I'll try to move him on with my leg, and if he doesn't respond appropriately then he gets tapped with the whip. If he doesn't buck then that's great. If he does buck then he will get spun around, tapped again, and MADE to go. . If he tries again, then it gets worse. Even after the issue has been resolved I will still touch him with the whip (remember, I'm not smacking the horse with the whip or anything) just to push the issue so to say and make sure that he isn't going to try anything else. After the "blow up" he is always a perfect saint for the entire remainder of the ride- if I say "Jump" he says "How high??" (not literally because I rarely jump, but you get the idea...) and the whip hardly needs to be touched.
With this kind of handling of the situation however, you need to be prepared for the response that the horse will give you. If you don't have the confidence to handle the horse's response, then it is best to have a more experienced and confident rider go through some training on the horse so that you can build yours, and so the bucking becomes less frequent. Assuming that you have ruled out causes of pain, in my world bucking is something that is unacceptable and should not be tolerated. My guess that she is just ticked off at the whip since the bucks directly correspond with the whip, but all other causes should be ruled out as well!