What do I do with a 2yr.old filly that shakes her head and bucks? I am using a hackamore on her. I have been riding her with no trouble up until now.
Up until now you have had no trouble with your filly, and you are using a hackamore on her. My suggestion would be to begin with having your vet check and float her teeth. Although you are not using a bit inside her mouth, the hackamore surrounds her muzzle and can cause the teeth to rub on the cheeks and cause pain. Your filly may have hooks on some of her cheek teeth that are cutting into her cheek tissues, causing the head shaking, followed by her bucking. While at the vet, I would also have him/her check her back for soreness, which could also be why she has been agreeable up to now.
Thank You and Happy Trails!