Storm Crow gave you our standard "educated people's" answer here: Only a doctor who can evaluate you in person and talk to you in detail about the pain can tell you what it is. It could be anything from a sore muscle to an irritated place in your lungs to anxiety to bronchitis to a "stitch" in your side to something like a small area of pleurisy or atelectasis. A doctor or nurse practitioner can palpate, take an X-ray, check your oxygen saturation, and put all the info he/she obtains together and hazard an educated guess. No one with any sense would even attempt that over the Web.