Diaphragm refers pain to the shoulder. Regardless, my bet would be musculoskeletal because you've been working out. Probably costochondritis or an intercostal or serratus anterior muscle strain. Unfortunately the only cure is rest. Of course, being on the left side makes me thing of possible cardiac (e.g. pericarditis), or of course a pulmonary etiology is always possible. Key question is whether or not the pain is reproducible on palpation (i.e. pushing where it hurts). Use the flat of your hand and moderate pressure (you can make anything hurt if you jab it with your fingers hard enough

). If not, or if it worsens or changes, definitely seek professional opinion. In fact, if you haven't seen your neighborhood doc recently or never had an EKG before you might want to go for a routine health visit and have it checked out.