The causes of sudden and severe mid back pain are many and varied. Some of the many things that can cause those symptoms are infections, rib and cartilage dysfunction/irritation, idiopathic (unknown cause) muscle spasms, and vertebral disc herniation, along with many other disease processes.
The things to think about are 1) what were you doing when the pain started? 2) is this the first time it's ever happened or is it a recurrence? 3) is the pain related to your breathing patterns? 4) is the pain related to specific movements?
Unfortunately there is no easy answer without some type of examination.
If you are running a fever, feel faint, or have shortness of breath you need to see your medical doctor.
If your pain is caused with certain movements, hurts when you breathe (rib pain), or if you feel that your pain is caused by some type of neuromusculoskeletal (nerve, muscle, and bone) problem, then you are probably better off finding a reputable chiropractor.