If she's new to you don't judge her on anything yet...it took me a solid week before my mare stopped trying to buck me off after I bought her. And I mean every single second that passed with me on her back she was bucking, at the halt, walk, trot, canter, didn't matter. Mares especially (In my experience) will test their riders. Geldings do it too of course, but I think mares REALLY make you work for your position as their leader. Stick with it...horses test riders, and not only that, she may be nervous/scared/unsettled in her new place and with her new leader.
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.