Viral Gastroenteritis: This is the most common cause of diarrhea and stomach pain, often due to norovirus or rotavirus, and is usually self-limiting. The symptoms typically include watery diarrhea, abdominal cramps, and sometimes vomiting.

Food Poisoning: Bacterial or parasitic infections from contaminated food can cause diarrhea, stomach pain, and vomiting. Common culprits include Salmonella, E. coli, and Campylobacter.
Irritable Bowel Syndrome (IBS): A chronic condition characterized by recurring abdominal pain and changes in bowel movements, which can include diarrhea.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, and weight loss.
Lactose Intolerance: The inability to digest lactose can lead to bloating, gas, diarrhea, and stomach cramps after consuming dairy products.

Appendicitis: Although less common, appendicitis can present with abdominal pain and diarrhea. If missed, it can lead to perforation and peritonitis, which are life-threatening.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause severe abdominal pain, vomiting, and bloody stools. It requires immediate medical attention.
Mesenteric Ischemia: A reduction in blood flow to the intestines can cause severe abdominal pain, diarrhea, and potentially lead to bowel infarction if not promptly treated.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, abdominal pain, weight loss, and malabsorption.
Giardiasis: An intestinal infection caused by the Giardia parasite, which can lead to chronic diarrhea, abdominal cramps, and weight loss.
Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine, causing diarrhea, abdominal pain, and malabsorption.
Tropical Sprue: A rare condition characterized by chronic diarrhea, abdominal pain, and malabsorption, typically acquired in tropical regions.