Gastroenteritis: This is often the most common cause of abdominal pain and nausea, typically resulting from a viral or bacterial infection. The symptoms can range from mild to severe and usually include diarrhea and vomiting.

Irritable Bowel Syndrome (IBS): A chronic condition characterized by recurring abdominal pain associated with a change in bowel habits, such as diarrhea or constipation. Nausea can also be a symptom.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause abdominal pain, nausea, diarrhea, and weight loss due to chronic inflammation of the gastrointestinal tract.
Peptic Ulcer Disease: Ulcers in the stomach or duodenum can cause abdominal pain, nausea, and sometimes vomiting. The pain is often described as burning and can be relieved by eating or taking antacids.
Appendicitis: Although it typically presents with localized right lower quadrant pain, early stages can have more generalized abdominal pain and nausea. It's a significant cause of acute abdominal pain.

Acute Mesenteric Ischemia: A life-threatening condition where the blood flow to the intestines is blocked, leading to severe abdominal pain out of proportion to the physical examination, nausea, and possibly vomiting. Early diagnosis is crucial.
Ectopic Pregnancy: In women of childbearing age, ectopic pregnancy can present with abdominal pain and nausea, and it is a medical emergency.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain radiating to the back, nausea, and vomiting. It requires prompt medical attention.
Intestinal Obstruction: A blockage in the intestine can lead to severe abdominal pain, nausea, vomiting, and constipation. It's a surgical emergency.

Porphyrias: A group of disorders that can cause acute intermittent abdominal pain, nausea, and neurological symptoms due to defects in heme biosynthesis.
Celiac Disease: An autoimmune reaction to gluten can cause abdominal pain, nausea, diarrhea, and malabsorption.
Whipple's Disease: A rare bacterial infection that affects the small intestine, causing malabsorption, weight loss, abdominal pain, and nausea.
Abdominal Migraine: A condition that causes recurrent episodes of severe abdominal pain, often accompanied by nausea and vomiting, similar to migraine headaches but without the headache.