Gastroenteritis: This is the most likely diagnosis given the acute onset of abdominal pain and diarrhea. The high pain rating (8/10) and the presence of diarrhea for 24 hours suggest a viral or bacterial infection of the gastrointestinal tract, which is commonly seen in gastroenteritis. Treatment typically involves fluid replacement, rest, and sometimes antiviral or antibiotic medications depending on the cause.

Irritable Bowel Syndrome (IBS): Although IBS is a chronic condition, an acute exacerbation could present with severe abdominal pain and diarrhea. The diagnosis is based on the Rome IV criteria, which include recurrent abdominal pain associated with defecation and changes in stool frequency or form. Treatment involves dietary changes, stress management, and sometimes medications to manage symptoms.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with abdominal pain and diarrhea. These are chronic conditions, but an acute flare could mimic the symptoms described. Diagnosis involves endoscopy, biopsy, and imaging studies. Treatment typically involves immunosuppressive medications and sometimes surgery.
Food Poisoning: This is another common cause of acute abdominal pain and diarrhea, resulting from the consumption of contaminated food. The diagnosis is based on history and sometimes confirmed by stool tests. Treatment is usually supportive, focusing on hydration and sometimes antimicrobial therapy.

Appendicitis: Although the presence of diarrhea might be atypical, appendicitis is a surgical emergency that cannot be missed. It typically presents with right lower quadrant abdominal pain, but the pain can be diffuse or accompanied by other symptoms like nausea, vomiting, and changes in bowel habits. Diagnosis is often made clinically and confirmed with imaging studies like CT scans. Treatment involves surgical removal of the appendix.
Intestinal Obstruction: This condition can cause severe abdominal pain and changes in bowel habits, including diarrhea in some cases. It is a medical emergency requiring prompt diagnosis and treatment, often surgical. Diagnosis involves imaging studies like X-rays and CT scans.
Mesenteric Ischemia: This is a condition where there is insufficient blood flow to the intestines, which can cause severe abdominal pain out of proportion to the physical examination findings. It is more common in older adults and those with cardiovascular disease. Diagnosis involves imaging studies like CT scans with angiography. Treatment often requires surgery and sometimes endovascular interventions.

Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, weight loss, abdominal pain, and diarrhea. Diagnosis is made by small intestine biopsy. Treatment involves long-term antibiotic therapy.
Eosinophilic Gastroenteritis: A rare condition characterized by the infiltration of eosinophils into the gastrointestinal wall, leading to abdominal pain, diarrhea, and sometimes intestinal obstruction. Diagnosis involves endoscopy with biopsy, and treatment often includes corticosteroids.
Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to abdominal pain, diarrhea, and weight loss. Diagnosis is made by stool tests, and treatment involves antimicrobial therapy.