Gastroesophageal Reflux Disease (GERD): This condition is characterized by stomach acid frequently flowing back into the tube connecting the mouth and stomach (esophagus). This backwash (acid reflux) can irritate the lining of the esophagus, causing discomfort. Pain from GERD can be felt in the chest and upper abdomen, potentially radiating to the left side, especially if the pain is related to eating or lying down.

Peptic Ulcer Disease: Ulcers in the stomach or the first part of the small intestine (duodenum) can cause pain that may be felt in the mid-abdomen and can radiate to the left side, depending on the ulcer's location. The pain is often described as burning and can be related to eating.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain that typically radiates to the back. While the pain is usually more central, it can be felt on the left side, especially if the tail of the pancreas is involved.
Diverticulitis: Inflammation of the diverticula (small pouches) in the wall of the colon can cause pain, usually on the left side of the lower abdomen. However, if the inflammation is higher up in the colon, the pain could be perceived more centrally or even on the left side near the rib cage.
Kidney Stones: While kidney stones typically cause flank pain, if a stone is moving down the ureter, it can cause severe pain that might be felt more centrally or even radiate to the abdomen.

Myocardial Infarction (Heart Attack): Although more commonly associated with chest pain, a heart attack can sometimes present with pain that is perceived in the abdomen, especially if the inferior wall of the heart is involved. Missing this diagnosis can be fatal.
Pulmonary Embolism: A blood clot in the lungs can cause sudden onset of chest pain that might be perceived on the left side, especially if the clot is located in the left lung. This condition requires immediate medical attention.
Aortic Dissection: A tear in the inner layer of the aorta can cause severe, tearing chest pain that can radiate to the back and potentially be felt in the abdomen. This is a medical emergency.

Splenic Infarction: The spleen's blood supply can become blocked, leading to tissue death. This condition can cause severe left upper quadrant abdominal pain.
Left Lower Lobe Pneumonia: Infection in the left lower lobe of the lung can cause pain that is referred to the left side of the abdomen due to the shared nerve roots.
Intestinal Obstruction: A blockage in the intestine can cause severe abdominal pain, vomiting, and constipation. The pain can be diffuse or localized, depending on the obstruction's location.