What could be the cause of pain in the upper left side of my back?

Family practice . Probable muscle strain. Treat with NSAID . Ice/heat , then physical therapy if needed. Rest from heavy lifting for a few weeks.
Muscular. Self or profess massage, chiropractor, yoga, heat, stretching all are needed. My favorite in my practice is acupuncture and myofascial tissue release. Be careful some docs will place the blame in you disc and advise surgery or epidurals .... No!