Horse's Back End is Stiff

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!
Gail

Have you has her checked out by the vet?
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Kissing spine comes to mind, similar to low back pain in people. A vet needs to make a diagnosis.

with that would she be able to just not buck at all when I'm on her because occasionaly someone else does and she bucks like crazy and she never has with me. Her canter is always really bumpy and stuff but...

No not yet I wanted to see what you guys said first

I would consult a vet first, online forums second.
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I will I just didn't know if it was just something that I shouldn't worry about or not...
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