OP - Pain sounds like a very good option. And the second, is her throwing a temper tantrum because she doesn't respect you/didn't understand what you were asking since she's young and green.
Either way, you should seek professional help. Have the vet out, have saddle fit checked, and ask a trainer to help you. My 3 year old bucked once out of a temper tantrum/repsect issue because she didn't want to go forward, but I made her do it anyway and we haven't had an issue since. If you allow her to keep bucking, and bucking you off, she will learn that if she bucks and you fall off, it's a way to get out of work. I wouldn't allow that to become a habit.