I have got upper back pain left side what's the problem?

Need an Evaluation: Could be several things: muscle spasm, kidney disease, kidney disorder, lung, ribs, spine.. An exam and a conversation will help narrow the field of causes.

Big Question: Due to the upper back containing so many structures.....A ton of muscles and the nerves that serve them, underlying bones, spinal column, lungs....You should not guess what your problem is, but see someone who will work this up.

Muscle strain: Muscle strain. Use flexaril or skelaxin (metaxalone) with moist heat to the area. Pulmonary embolism ie blood clot to the lung is much less likely.