It sounds like she is just back sore. I would still have the other vet come and see what he / she says, but without seeing your horse it sounds like her back was injured. Could be she pulled a muscle slipping or running around, her conformation, or the fact she is ridden bareback. Most horses are fine with being ridden bareback, but perhaps in a combination of a long back or being younger, the weight in one spot or you bouncing around a little bit (not saying you are a bad rider, but everyone bounces some bareback!) could be too much strain on her back.
Casey went a little back-sore after a long backpacking trip and we brought her to the chiropractor a couple weeks later and after she worked on her, Casey was perfectly sound, and happy! I believe it was a combination of 5+ hour rides (with a couple breaks) each day, and her longer back.
Edit - getting a good liniment, I love sore-no-more, and rubbing it on her back might help, as well as cold hosing if it isn't too cold out there.
“Good things come to those who wait… greater things come to those who get off their *** and do anything to make it happen.” - Unknown