Gastroenteritis: This is a common condition characterized by inflammation of the stomach and intestines, often due to viral or bacterial infections. Symptoms include severe stomach pain, vomiting, and sometimes diarrhea. The location of the pain under the ribs and the presence of vomiting align with gastroenteritis, especially if the patient has recently been exposed to someone with similar symptoms or has consumed contaminated food or water.

Peptic Ulcer Disease: Peptic ulcers can cause severe stomach pain, often described as a burning sensation that can radiate to the back. Vomiting may occur, especially if the ulcer is bleeding or if there is an obstruction. The pain is typically localized to the upper abdomen and can be exacerbated by eating or relieved by antacids.
Pancreatitis: Inflammation of the pancreas can cause severe, persistent pain in the upper abdomen that radiates to the back. Vomiting and nausea are common, and the condition can be acute or chronic. The pain under the ribs could be consistent with pancreatitis, especially if there's a history of gallstones or excessive alcohol use.
Cholecystitis: Inflammation of the gallbladder, often due to gallstones, can cause severe right upper quadrant abdominal pain that may radiate to the right shoulder or back. Vomiting and fever may also be present. The pain's location under the ribs, particularly on the right side, could suggest cholecystitis.

Appendicitis: Although appendicitis typically presents with pain in the lower right abdomen, it can occasionally cause upper abdominal pain, especially in the early stages. Vomiting may occur, and it's crucial not to miss this diagnosis due to the risk of the appendix rupturing, which is a medical emergency.
Myocardial Infarction (MI): In some cases, a heart attack can present with epigastric discomfort or pain that may be mistaken for a gastrointestinal issue. Vomiting can occur, especially if the patient is experiencing significant stress or pain. It's critical to consider MI, especially in patients with risk factors for heart disease.
Pulmonary Embolism: While less common, a pulmonary embolism can cause sudden onset of chest pain that might be referred to the upper abdomen, along with vomiting if the patient is experiencing significant distress or decreased oxygenation.

Hepatic Abscess: A rare condition where an abscess forms in the liver, which can cause severe right upper quadrant pain, fever, and vomiting. The pain could be referred to the area under the ribs.
Splenic Infarction or Rupture: Conditions affecting the spleen, such as an infarction (death of splenic tissue due to lack of blood supply) or rupture, can cause severe left upper quadrant pain that may radiate to the left shoulder or back, along with vomiting. These conditions are less common but can be life-threatening.
Gastric Volvulus: A rare condition where the stomach twists abnormally, which can cause severe epigastric pain, vomiting, and sometimes difficulty swallowing. The pain could be perceived under the ribs due to the stomach's abnormal position.