Musculoskeletal strain: This is often the most common cause of abdominal side pain, resulting from overuse or injury to the muscles in the abdominal wall. The pain is typically sharp and localized to one area.

Kidney stones: Pain from kidney stones can radiate to the sides of the abdomen, especially if the stones are moving down the ureters. The pain is often severe and may be accompanied by nausea, vomiting, and blood in the urine.
Urinary tract infection (UTI): While UTIs more commonly cause pain in the lower abdomen or pelvic area, they can also cause flank pain that radiates to the sides of the abdomen, especially if the infection has moved up to the kidneys.
Constipation: Severe constipation can cause abdominal pain that may be felt on the sides, due to the buildup of stool in the intestines and the subsequent pressure on the abdominal wall.

Appendicitis: Although appendicitis typically causes pain in the lower right abdomen, it can sometimes cause pain on the sides if the appendix is located in a retrocecal position. Missing this diagnosis can lead to perforation and severe complications.
Ectopic pregnancy: In women, an ectopic pregnancy can cause severe abdominal pain, including pain on the sides, and is a medical emergency that requires immediate attention.
Pulmonary embolism: In rare cases, a pulmonary embolism can cause referred pain to the abdomen, including the sides, especially if the embolism is large and causes significant strain on the lungs and diaphragm.

Inflammatory bowel disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic abdominal pain, including pain on the sides, although this is less common.
Abdominal aortic aneurysm: A large aneurysm can cause pain in the abdomen, including the sides, due to pressure on surrounding structures, although this is more typically felt in the mid-back or lower abdomen.
Pancreatitis: Pancreatic inflammation can cause severe abdominal pain that may radiate to the sides, although this is more commonly felt in the upper abdomen.