Pericarditis: This condition is characterized by inflammation of the pericardium, which can cause chest pain that worsens with deep inspiration and when laying flat. The recent history of strep throat, which was treated with antibiotics, could be a precipitating factor. The fact that fevers have resolved and there are no significant ECG findings does not rule out pericarditis, especially if it's in the early stages or not severe enough to cause significant ECG changes.

Precordial Catch Syndrome: This is a common cause of chest pain in children and adolescents, characterized by sharp, stabbing pain that can worsen with deep breathing or movement. It's often benign and related to the musculoskeletal structures of the chest wall.
Costochondritis: Inflammation of the cartilage that connects the ribs to the breastbone can cause chest pain that worsens with deep inspiration and movement. It's a common condition in children and can be related to recent infections or trauma.
Musculoskeletal Pain: Given the recent illness and potential for coughing or straining, musculoskeletal pain from strained chest muscles or ribs could be a consideration.

Pulmonary Embolism: Although rare in children, pulmonary embolism can present with chest pain that worsens with deep inspiration. It's crucial to consider this diagnosis, especially if there's any history of recent immobilization, family history of clotting disorders, or other risk factors.
Myocarditis: Inflammation of the heart muscle can occur after a viral infection and can present with chest pain, among other symptoms. It's essential to consider this diagnosis given the recent history of strep throat.
Pneumonia: Despite the resolution of fevers and no shortness of breath, pneumonia could still be a consideration, especially if the chest pain is related to a recent respiratory infection.

Spontaneous Pneumothorax: Although more common in tall, thin males, spontaneous pneumothorax can occur in anyone and would present with sudden onset chest pain that worsens with deep inspiration.
Thoracic Outlet Syndrome: Compression of the nerves or blood vessels between the collarbone and first rib can cause chest pain and other symptoms, but it's relatively rare in children.
Tietze's Syndrome: A rare condition characterized by inflammation of the cartilages and surrounding tissues of the ribs, which can cause chest pain similar to costochondritis but is less common.