I have an older gelding who has issues on the hind end ... lots of arthritis, some ringbone. He's actually retired from riding now. But in the last couple of years of riding, he would be fine many days, but on some he would throw himself sideways. I always interpreted it as "protecting himself," meaning there was a physical reason for it.
I hate to say it, but perhaps the best way to determine how much is to have a vet try to eliminate the pain with drugs and see what changes. Sounds like the location may not warrant a standard "blocking" approach (start low, keep going higher), but something more general.
But be forewarned if you go that route, her response may not be to just plain go straight like you want, but to go completely batty with joy over the removal of pain.