The pain under the right side of the rib cage on the first breath is mainly due to three factors. The first is chronic strain, acute injury, and infection of the ribs, intercostal muscles, and intercostal nerves themselves, resulting in a local inflammatory response that strains the diseased area during respiratory movements and produces pain. The second is the disease of the subcostal pleura. The common one is pleurisy, where the presence of inflammation in the pleura causes pain in the two layers of the pleura stimulated by inflammation with respiratory movements. The third, inflammation of the peritoneum. Common diseases of peritoneal inflammation include right-sided subcostal cholecystitis, cholelithiasis, hepatic cysts, and other diseases. Due to the inflammation of the peritoneum caused by the stimulation of these diseases, pain is produced by the up and down movement of the diaphragm pulling the inflamed peritoneum when breathing movement. A detailed examination at the hospital is recommended in this case.