The middle of your back would be around the level of the diaphragm, maybe the lower
part of the lungs, maybe the kidneys. Pain
can be due to anything. Probably, most often it is musculoskeletal. Bilateral back pain would be consistent with a muscle strain or maybe a nerve or disc problem. Back pain is like a headache -- so many things can cause it that you need to be examined by a doctor, in order to find out what is the problem.