I was diagnosed with fibromyalgia in 2011, one of my worst symptoms was upper back pain, right in between my shoulder blades. Not even massages helped. In my experience, it was due to tension. I realized that my shoulders were often scrunched up. Even though it was hard to do at first because I was so tense, yoga helped immensely. Google 'yoga poses for back pain' and there's a lot of good videos that can help. I also had remember that I needed to relax, and when I felt that sharp pain between my shoulders I would take a deep breath and relax my shoulders. It takes time, but helps. Good luck.