Viral Gastroenteritis: This is the most common cause of vomiting in infants, characterized by a sudden onset of symptoms, including vomiting, which can be yellow due to the presence of bile. The fact that the child has maintained fluid intake and appears well at triage, with normal voiding and bowel movements, supports this diagnosis.

Overfeeding or Feeding Too Quickly: This can lead to vomiting, especially if the infant is being fed too much or too fast, causing stomach discomfort.
Gastroesophageal Reflux Disease (GERD): Although more common in infants with a history of spitting up, GERD can cause vomiting, which may be yellow.
Food Allergy or Intolerance: An allergic reaction or intolerance to a particular food can cause vomiting, though this is less common in infants who are exclusively breastfed.

Pyloric Stenosis: A condition where the pylorus (the passage between the stomach and small intestine) thickens, causing projectile vomiting. Although the vomiting described is not projectile, this condition is critical to rule out due to its potential for severe dehydration and electrolyte imbalance.
Intussusception: A condition where a part of the intestine slides into another, causing bowel obstruction. Vomiting can be a symptom, and it's crucial to consider this diagnosis due to its potential for severe complications if not promptly treated.
Infections (e.g., Urinary Tract Infection, Meningitis): Although the child appears well, infections can sometimes present with nonspecific symptoms like vomiting in infants.

Metabolic Disorders: Certain metabolic disorders can cause vomiting, among other symptoms. These are rare but important to consider if the child does not respond to typical treatments for more common conditions.
Congenital Anomalies of the Gastrointestinal Tract: Conditions like duodenal atresia or stenosis can cause vomiting, especially if it's bilious (yellow or green), indicating an obstruction past the ampulla of Vater.