Gastroenteritis: This is the most likely diagnosis given the acute onset of nausea, vomiting, diarrhea, and blood in the stool. The presence of blood could indicate an infectious cause, and the lack of fever does not rule out this diagnosis since some cases of gastroenteritis can be afebrile.

Inflammatory Bowel Disease (IBD) Flare: Although the patient has no significant history, it's possible that this could be the initial presentation of an IBD like ulcerative colitis or Crohn's disease. The presence of blood in the stool supports this consideration.
Food Poisoning: Despite the patient reporting no new food, food poisoning remains a possibility, especially if the food was contaminated. The symptoms align with food poisoning, which can cause nausea, vomiting, diarrhea, and sometimes blood in the stool.
Intestinal Infection: An infection caused by bacteria, viruses, or parasites could explain the symptoms. The absence of fever and recent travel or food history does not exclude this possibility entirely.

Intussusception: Although rare in adults, intussusception can cause abdominal pain, nausea, vomiting, diarrhea, and bloody stools. It's a medical emergency requiring prompt diagnosis and treatment.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can present with abdominal pain, nausea, vomiting, and diarrhea. It's a critical condition that requires immediate attention.
Diverticulitis: Given the patient's age, this might be less common, but it cannot be entirely ruled out, especially if the patient has a history of diverticula. The presence of blood in the stool and the potential for complications like abscess or perforation make it a "do not miss" diagnosis.

Ischemic Colitis: This condition, caused by insufficient blood flow to the colon, can present with similar symptoms. It's less common but should be considered, especially if other diagnoses are ruled out.
Colonic Angiodysplasia: A vascular anomaly of the colon that can cause bleeding. It's rare and typically found in older adults but could be considered in the differential diagnosis if common causes are excluded.
Colorectal Cancer: Although extremely rare in a 31-year-old, if the patient has a significant family history or other risk factors, it might be worth considering, especially if the bleeding persists or other symptoms develop.