Back pain in the middle of my back sharp?

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.
Depend on your age. Could be disc, or fracture vertebra, or muscular from scoliosis , or throracic radiculitis like before singles, or diabetes, etc--ask for good neurological exam.