Long shot on relevance, but I did have similar, occasionally acute pain in my back the first few months after Whipple (open).

I've had a chronic bad back, but my Whipple week in an uncomfortable hospital bed was the initial trigger for it getting worse. Even after I got home and into a good bed, the "damage" was already done. Several weeks of sleeping in odd positions to alleviate pain where the Whipple incision was healing piled on top of that. It was worrisome for a while because it was the same area where back pain is often reported prior to a pancreatic cancer diagnosis.

I was lucky enough to discover that it was all muscular. Massage and physical therapy managed to untangle all the knots in my mid-back muscles and provide some relief. I hope you're able to find an equally easy fix, but if massage/PT don't provide it, you'll probably still feel better for giving it a try. 🙂