Sudden upper back pain and hurts when i breathe in?

Doctor can evaluate: Several possible reasons in a young adult, but one's doctor may want to check for a collapsed lung (pneumothorax, due to air leaking from inside the lung through it's lining, out into the space between the lung and the ribs). The site of the leak has some damage, which may heal itself. The leaked air stays inside the rib cage and takes up space, preventing the lung from fully inflating.