Gastroesophageal Reflux Disease (GERD): This condition is common in infants and can cause projectile vomiting, especially after feeding. The absence of pyloric stenosis on ultrasound and the age of the infant make GERD a likely diagnosis.

Overfeeding or Incorrect Feeding Technique: Overfeeding or using an incorrect feeding technique can lead to vomiting in infants. This is a common issue that can be addressed through counseling on proper feeding techniques.
Lactose Intolerance or Milk Protein Allergy: Although less common, lactose intolerance or a milk protein allergy could cause vomiting in an infant. These conditions would require dietary changes.
Infection (e.g., Urinary Tract Infection, Gastroenteritis): Infections can cause vomiting in infants. A thorough physical examination and appropriate diagnostic tests (e.g., urine analysis, stool tests) can help identify an infectious cause.

Intestinal Obstruction: Although the ultrasound was negative for pyloric stenosis, other forms of intestinal obstruction (e.g., duodenal atresia, intestinal malrotation) need to be considered. These conditions are surgical emergencies.
Congenital Adrenal Hyperplasia: This condition can cause vomiting due to salt wasting and is life-threatening if not promptly diagnosed and treated.
Inborn Errors of Metabolism: Certain metabolic disorders can present with vomiting in the neonatal period. Early diagnosis is crucial for management and prognosis.

Eosinophilic Esophagitis: A rare condition characterized by eosinophilic infiltration of the esophagus, leading to vomiting and feeding difficulties.
Antral Web or Other Rare Anatomical Abnormalities: These are congenital anomalies that can cause gastric outlet obstruction and vomiting. They are less common than pyloric stenosis but should be considered in the differential diagnosis.
Neonatal Pylorospasm: A condition where the pylorus muscle goes into spasm, causing intermittent gastric outlet obstruction. It's less common and can be difficult to diagnose.