Hi thanks for asking question. Noted you have bronchitis plus history of MI attack also. So you are already respiratory and cardiac compromised.Whenever you do some more activity then your tissue might not getting enough oxygen as cardiorespi compromise. Here you have to do ECG and echo cardiography to rule out cardiac failure secondary to MI.As in cardiac failure also dyspnea and chest pain can occur. If bronchitis is infective then treated with antibiotic. If more difficulty in breathing and chest pain at rest then consultation to doctor done and nebulization with subsequent oxygen therapy might needed. So better to consult cardiologist to rule out heart failure. Avoid smoking if habit. Take care...