What could cause the left side of my chest straight through to my back hurt? It hurts when I breathe, cough, sneeze.... it's just painful

Several Things. Left-sided chest pain that radiates through to your back includes, from less to more concerning, gas and indigestion, musculoskeletal strain or inflammation, pneumonia, collapsed lung, pancreatitis, heart attack and aortic dissection. In the setting of recent trauma, splenic rupture would be considered also. Depending on your age and risk factors, any of these become more or less likely.