Sharp pain when lying on left side. approx halfway up ribs. no pain in any other position. instantly relieved by moving off left side.

Because this pain is: SO positional, I have to suspect a musculoskeletal cause such as rib contusion or fracture, intercostal muscle strain, etc. Pleurisy, inflammation of lung lining, or pericarditis would likely hurt with other positions and activity (deep breathing, coughing).Perhaps you can recall some chest trauma? I think you can watch it; it should gradually improve. See Doc if it doesn't or more symptoms appear