Gastroenteritis: This is a common condition that can cause abdominal pain, diarrhea, and sometimes a rash, especially if the causative agent is a virus or bacteria that also triggers an allergic or immune-mediated skin reaction.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic abdominal pain, diarrhea, and may be associated with extraintestinal manifestations including skin rashes.
Food Allergy or Intolerance: Certain food allergies or intolerances can lead to abdominal pain, diarrhea, and skin rashes due to an immune reaction or malabsorption.
Irritable Bowel Syndrome (IBS): While IBS primarily causes abdominal pain and altered bowel habits, some patients may experience diarrhea and, less commonly, skin manifestations due to associated stress or other factors.

Septicemia or Bacteremia: Certain bacterial infections can cause abdominal pain, diarrhea, and a rash, and are life-threatening if not promptly treated.
Meningococcemia: A severe infection caused by Neisseria meningitidis, characterized by a rash, and can initially present with nonspecific symptoms like abdominal pain and diarrhea.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Severe skin and mucous membrane disorders usually caused by medications, which can also involve the gastrointestinal tract and present with abdominal pain and diarrhea.

Whipple's Disease: A rare, systemic bacterial infection that can cause abdominal pain, diarrhea, and skin rash, among other symptoms.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues, which can lead to gastrointestinal symptoms and skin manifestations.
Sjögren's Syndrome: An autoimmune disorder that can cause a variety of symptoms including gastrointestinal issues and skin rashes, though it is more commonly associated with dry eyes and mouth.