Nasal congestion or upper respiratory infection: This is a common condition in infants that can lead to increased respiratory effort, causing intercostal and sternal indrawing due to the use of accessory muscles for breathing.

Gastroesophageal reflux disease (GERD): GERD can cause discomfort and lead to increased respiratory effort, especially after feeding, resulting in indrawing.
Pneumonia or bronchiolitis: These lower respiratory tract infections can cause increased work of breathing, leading to the observed signs.
Anemia: Anemia can lead to increased respiratory rate and effort, causing indrawing.

Congenital heart disease: Conditions like tetralogy of Fallot or hypoplastic left heart syndrome can present with signs of increased respiratory effort due to heart failure or decreased pulmonary blood flow.
Pneumothorax: Although less common in this age group without trauma, a spontaneous pneumothorax could cause sudden onset of respiratory distress and indrawing.
Diaphragmatic hernia: A congenital diaphragmatic hernia can cause respiratory distress and indrawing due to herniation of abdominal contents into the thoracic cavity.

Cystic fibrosis: An early presentation of cystic fibrosis could include respiratory symptoms due to thickened secretions, although this would typically be accompanied by other symptoms like failure to thrive or gastrointestinal issues.
Interstitial lung disease: Rare conditions affecting the lung interstitium could present with respiratory distress and indrawing.
Skeletal dysplasias: Certain genetic conditions affecting bone growth could lead to a small thoracic cavity and increased respiratory effort, although these would typically be identified prenatally or at birth.