What can cause pain in the upper left part of my back?

Many things. Although by far the most common cause is muscular in nature, other causes include heart disease (coronary artery disease), lung disease and spinal disorders, to name just a few. If the pain has been persistent, it is best to be evaluated by your personal family physician. Another option is to see a physical therapist who should be able to tell you if it is simply musculoskeletal in nature.