Woke up with pain in one area on the left side of back, possibly behind the lungs or heart. What could this be? Pneumonia or inflammation?

Pain. Pain is caused by many things. More information would be necessary to postulate the cause of your pain. The most likely, however, with no more information is musculoskeletal pain. Having said that, however, with no more information the possibilities are legion.
Back pain. there is a lot of possibilities here: left sided pain can be associated with lung infections such as pleuritic or pneumonia, spleen problems or left sided kidney infection. Cardiac conditions less likely at you age of 19, but pericarditis is also a possibility, please see your doctor asap.