Just a thought from previous experience, I might be suspicious of her back. Bulging disc, etc. It can happen suddenly and be hard to diagnose, sometimes requiring an MRI. I hope it's not a disc, but I would want to rule that out.
Best thoughts to you and her!
'If you find a way to enjoy the ride, it doesn't really matter where you end up'
5 GSDs at the gate: Katie, Iroc, Red, River, Jake