It sounds like you are using a single joint snaffle? If so it is probably jabbing her in the roof of her mouth the causing the head coming up, she's trying to find relief from the nutcracker action. Also, Dr. Bristol's are actually not considered mild because of how the middle piece sits. The angle that is it at causes it to "dig in". I would steer clear of them, go with a french link snaffle, the cheekpiece is up to you and what your horse needs.
Sully ~Sullivan's Fly Supply~ [17.1 TB] RIP 2/24/14
Rio ~Camperio~ [18.0 Oldenburg]