Gastroenteritis: This is the most likely diagnosis given the symptoms of diffuse abdominal pain, vomiting, and loose stools. Gastroenteritis, often caused by viral or bacterial infections, is a common condition that matches the patient's presentation.

Food Poisoning: Similar to gastroenteritis, food poisoning can cause severe abdominal pain, vomiting, and diarrhea. It is usually caused by consuming contaminated food or drinks.
Irritable Bowel Syndrome (IBS): Although IBS is more chronic, an acute exacerbation could present with diffuse abdominal pain and changes in bowel habits, including loose stools and vomiting.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause abdominal pain, diarrhea, and vomiting, especially during flare-ups.

Appendicitis: Although appendicitis typically presents with localized right lower quadrant pain, it can initially present with diffuse abdominal pain. Missing this diagnosis can lead to severe consequences, including perforation and peritonitis.
Intestinal Obstruction: This is a surgical emergency that can cause severe abdominal pain, vomiting, and changes in bowel habits. Early diagnosis is crucial to prevent complications.
Mesenteric Ischemia: This condition, which involves insufficient blood flow to the intestines, can cause severe abdominal pain out of proportion to the physical examination findings, along with vomiting and diarrhea.

Whipple's Disease: A rare, systemic bacterial infection that can cause abdominal pain, diarrhea, weight loss, and arthralgias.
Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, leading to abdominal pain, vomiting, and diarrhea.
Celiac Disease: An autoimmune reaction to gluten, leading to small intestine damage and causing abdominal pain, diarrhea, and malabsorption.