Gastroenteritis: This is the most common cause of nausea and vomiting in infants, often caused by viral infections. The absence of fever does not rule out this diagnosis, as not all cases of gastroenteritis present with fever.

Food Intolerance or Allergy: Infants can develop intolerance or allergies to certain foods, leading to gastrointestinal symptoms like nausea and vomiting.
Overfeeding or Underfeeding: Improper feeding techniques or schedules can cause nausea and vomiting in infants.
Gastroesophageal Reflux Disease (GERD): GERD is common in infants and can cause nausea, vomiting, and discomfort after feeding.

Intussusception: A serious condition where a part of the intestine slides into another, which can cause vomiting, abdominal pain, and potentially lead to intestinal obstruction or perforation. Although less common, it's critical to consider due to its severe consequences.
Pyloric Stenosis: A condition characterized by the narrowing of the pylorus, the passage between the stomach and small intestine, leading to projectile vomiting. It typically presents in infants around this age.
Ingestion or Poisoning: Accidental ingestion of harmful substances can cause nausea and vomiting and is a critical diagnosis not to miss due to its potential for severe outcomes.

Congenital Anomalies of the Gastrointestinal Tract: Conditions such as duodenal atresia or stenosis, which can cause vomiting, are usually diagnosed earlier in life but could potentially present later.
Metabolic Disorders: Certain metabolic disorders can cause nausea and vomiting among other symptoms. These are rare but important to consider in the differential diagnosis.
Central Nervous System Causes: Conditions affecting the brain, such as hydrocephalus or brain tumors, can cause vomiting, though these would be extremely rare in this context and typically accompanied by other neurological signs.