Hi,
I am sorry you are struggling with your back. I used to have middle / upper back pain years ago, ways before my diagnosis, which sounds a bit like what you describe. At the time, I went to see an osteopath and I was given some stretches exercises to extend my spine a bit. So it seemed it was kind of posture related, but at the same time it could have been an early sign of AxSpa. Either way, for back pain, stretches tend to be quite good to ease the pain, as long as you are sure the stretches you are doing are adapted to you and your condition.
Have you had the chance to discuss this with a doctor or a physiotherapist? It might be good to get the view of a professional over this new symptom.
Also, have you found ways to ease your pain like heat pad, changing positions, gentle moves?
I hope you will get some pain relief soon.
Take care,
Julie.