I think pain is the issue.
You might find the information you need at http://thecorrector.net/id21.html. The whole site is interesting, this is just one page, and it is on how to find out if you horse's back is sore. Many of your horse's actions are described.
I have been using the Corrector for years now, both I and my riding teachers like it, and the horses relax their backs when I use it. I like it especially because I have poor balance from MS and it seems to protect the horse's backs from my sometimes extremely unsteady seat.
How does she act when you ride her bareback?
Please, be careful. Horses in pain are unpredictable.