Answer:
Chest pain can arise from anything in the chest fromt he skin to the muscles and other connective tissue of the chest wall to the organs such as the lung and its lining, the esophagus and the heart and the sack around it.
Sharp pains are usually from a problem with the muscles, tendons, ligaments etc or the lining of the lung. If it is mostly when you move the arm or torso is it probably from the chest wall while pain from the lung lining (pleurisy) usually causes pains with taking deep breaths.
Ocassionally, especially in women, sharp pains can be coming from the heart. If the pain is persistent (going on for more than a few days), severe or worse with exertion or accompanied by dizziness, shortness of breath, palpitations (rapid heart rates) or fever you should call your doctor.