Costochondritis: This is an inflammation of the cartilage that connects the ribs to the sternum. It is a common condition that can cause pain and warmth in the area under the sternum. The warmth is usually due to inflammation.

Gastroesophageal Reflux Disease (GERD): GERD can cause inflammation and irritation in the esophagus, which may lead to a sensation of warmth under the sternum, especially after eating.
Musculoskeletal Strain: Strain or injury to the muscles or cartilage in the chest area can lead to localized warmth due to increased blood flow to the area for healing.
Pericarditis: Inflammation of the pericardium, the sac surrounding the heart, can cause chest pain and possibly a sensation of warmth under the sternum.

Myocardial Infarction (Heart Attack): Although less common, a heart attack can present with atypical symptoms, including a feeling of warmth or discomfort under the sternum. It is crucial to consider this diagnosis, especially in patients with risk factors.
Pulmonary Embolism: A blockage in one of the arteries in the lungs can cause sudden chest pain and possibly a sensation of warmth, among other symptoms. This is a life-threatening condition that requires immediate medical attention.
Aortic Dissection: A tear in the inner layer of the aorta can cause severe, sudden chest pain that may be described as a feeling of warmth under the sternum. This is a medical emergency.

Thyroid Disorders: Certain thyroid conditions, such as thyrotoxic crisis, can cause widespread symptoms including chest discomfort or a sensation of warmth.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the chest, leading to unusual symptoms such as warmth under the sternum.
Tietze's Syndrome: A rare condition characterized by inflammation of the cartilages and surrounding tissues of the ribs, which can cause pain and possibly warmth in the chest area.