I lease a 17 yr old quarter horse mare at my barn (im not the only one riding her a walk/trotter rides her but I do walk/trot/canter) and she has become very very stiff in her back end. Both of us only ride her a total of 2 days a week. She has started bucking baddly when she canters and she can't jump anymore... We're not sure why she is... She bucks for everyone that canters her except me. Is there something I can buy to help her? What do you think it is? Thanks!