Multiple etiologies. The middle of the back is not a common place for disc disease, which is more common in the neck and the lower back. Depending on your age and your other medical problems, the pain could be a muscle spasm (eg from exercising), pain radiating from other places (e.g. Abdomen in case of pancreatitis), trauma to the back, compression fractures (e.g. In osteoporosis), and cancer with bone metatasis.