Viral Gastroenteritis: This is the most common cause of abdominal pain in infants, often accompanied by diarrhea and vomiting. The child's age and the prevalence of viral infections at this age make it a likely diagnosis.

Constipation: A common issue in infants, which can cause significant abdominal pain and discomfort.
Intussusception: Although more common in children around 6 to 12 months, it's a possibility in an 8-month-old, characterized by the telescoping of one portion of the intestine into another, leading to severe abdominal pain.
Gastroesophageal Reflux Disease (GERD): Can cause abdominal pain and discomfort in infants, often associated with feeding difficulties and regurgitation.

Intestinal Obstruction: Conditions like volvulus or intestinal atresia can cause severe abdominal pain and are life-threatening if not promptly diagnosed and treated.
Appendicitis: Although rare in infants, appendicitis can occur and requires immediate surgical intervention to prevent perforation and peritonitis.
Pyloric Stenosis: Typically presents with projectile vomiting but can also cause abdominal pain, and if missed, can lead to severe dehydration and electrolyte imbalances.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause abdominal pain, gastrointestinal bleeding, or obstruction, but is less common.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis are rare in infants but can present with chronic abdominal pain, diarrhea, and weight loss.
Abdominal Malignancies: Extremely rare in infants, but tumors like neuroblastoma can cause abdominal pain and should be considered in the differential diagnosis if other causes are ruled out.