I have mid to upper back pain. it feels as though my muscles never relaxed and feel strained. what could be wrong?

Myofascial pain: You may suffer from myofascial pain. It is caused by muscle strain and overuse and can occur after a neck or whiplash injury. Treatments like heat, massage, chiropractic, and stretching can help temporarily, but longterm relief hinges on strengthening the retractors of the shoulder blade, and correction of improper posture and muscle strength imbalances.

Chain reaction: Your entire spine is a series of curves. If one area's stuck or tight, another might shift or take on more loading that you feel as strain. See a doctor (do/md) skilled in rehab and osteopathic manipulative treatment to determine underlying medical concerns as well as biomechanical factors and treatment by them or by pt. Also find ways to relax your inner self as well as body.

Spasm: This question must be followed by a diagnosis from you doctor as these symptoms especially between the blades is almost always spasm usually from muscle stress. Sit a computer a lot? But pain has the potential to be caused from ton of causes from spasm to cancer. Check it out.