Sometimes this type of pain may indicate stiffness or "locking up" of the thoracic vertebrae in your mid-back. This may be due to lack of mobility in the mid back due to a possible posture imbalance or imbalance of the pushing muscles compared to pulling muscles.
Some things that may help is to focus on some lower trap and middle trap exercises to give more support to the shoulder blades and help them move more efficiently. Also, seated torso rotations and/or sidelying torso rotation stretches may help with the thoracic vertebrae mobility.

If your pain persists then consult your doctor, physical therapist which could probably help with this issue. Also, looking into Chiropractic may help with this if it is an issue of decreased mobility in your mid-back.

Hope this info helps

TM
www.AmpedNutrition.com