Gastroesophageal Reflux Disease (GERD): This is often the most common cause of stomach ache, characterized by symptoms such as heartburn and regurgitation, typically triggered by eating certain foods or lying down after meals.

Peptic Ulcer Disease: Includes both gastric and duodenal ulcers, which can cause a gnawing or burning pain in the upper abdomen, often relieved by eating or worse at night.
Gastritis: Inflammation of the stomach lining, which can be acute or chronic, leading to pain, nausea, and vomiting, often related to NSAID use, alcohol, or Helicobacter pylori infection.
Irritable Bowel Syndrome (IBS): A functional disorder of the bowel, leading to abdominal pain associated with changes in bowel habits, such as diarrhea or constipation, without an identifiable cause.
Food Poisoning or Gastroenteritis: Infections of the stomach and intestines, usually caused by bacteria, viruses, or parasites, leading to symptoms like diarrhea, vomiting, and abdominal cramps.

Appendicitis: Although the pain typically starts near the navel and then moves to the lower right abdomen, some cases can present with upper abdominal pain, making it crucial not to miss this diagnosis due to its need for urgent surgical intervention.
Myocardial Infarction: Chest pain can sometimes be referred to the abdomen, and in some cases, especially in women, the primary symptom of a heart attack can be epigastric discomfort rather than chest pain.
Pancreatitis: Inflammation of the pancreas, which can cause severe, persistent abdominal pain, often radiating to the back, and is associated with nausea, vomiting, and sometimes elevated pancreatic enzymes.
Cholecystitis: Inflammation of the gallbladder, usually due to gallstones, causing right upper quadrant pain, fever, and leukocytosis, which can sometimes be felt more diffusely in the abdomen.

Zollinger-Ellison Syndrome: A rare disorder characterized by excessive gastric acid secretion due to a gastrin-producing tumor, leading to severe peptic ulcers and diarrhea.
Celiac Disease: An autoimmune reaction to gluten, leading to inflammation and damage in the small intestine, which can cause abdominal pain, diarrhea, and malabsorption.
Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis): Chronic conditions that cause inflammation and irritation in the digestive tract, leading to symptoms like abdominal pain, diarrhea, weight loss, and fatigue.
Abdominal Aortic Aneurysm: A swelling of the main blood vessel that leads from the heart and passes through the abdomen, which can cause abdominal pain and is a medical emergency if it ruptures.