Chest pain when breathing hard may be caused by trauma, pneumonia, pleurisy or acute heart attack. 1. Trauma: If the chest has suffered trauma, such as rib fracture, soft tissue contusion, etc., it may lead to tissue damage or edema and congestion around the chest, which may cause pain by pulling the damaged area when breathing hard. 2. Pneumonia: If the lung infection is more serious, the lung parenchyma undergoes inflammatory changes, and the inflammation invades the pleura, which may lead to pain in the corresponding area when breathing hard. 3. Pleurisy: the patient’s lungs become infected under the influence of bacteria and viruses, resulting in pleurisy, which may cause chest pain when breathing hard under the stimulation of inflammation. 4. Acute infarction: If the patient’s cardiovascular embolism occurs, it may cause myocardial ischemia and hypoxia, which may lead to the symptoms of chest pain when breathing hard. If the above symptoms occur, it is recommended to go to the hospital in time for early and regular treatment.