It is difficult to speculate what may cause the condition without an assessment and inquiries about your medical and exercise history.
All the answers above may be possible ways to address your problem. My personal suggestion would be MELT (Myofascial Energetic Length Technique) which works on the fascia. I have seen encouraging results with upper back pain. I must confess, though, I am biased, since I am a MELT instructor.
What I recently did when I had pain that just wasn’t budging was a self-assessment. I did with myself what I would do with my clients: take pictures of static and dynamic posture and a video of the gait. As soon as I looked at all of it, I immediately got closer to the possible source. A very few exercises sufficed to determine the possible source of the imbalance, and I am now working on it.
Maybe that would be a way for you to make your own determination.