Hello dear Thanks for writing to us You did not mention the weather conditions, health history and history of smoking and drinking etc It looks like your vital capacity(capacity of lungs) is reduced and when you walk vigorously your body needs more oxygen and more energy. To combat the need of body heart pumps rapidly which causes increase in heart rate which you can feel at your pulse. Your alveoli(air bags) inside your lungs are more streesed to pump in oxgen along with accessory chest muscles which may cause mild chest pain and breathlessness on overexertion. So you need to get yourself examined by a chest physician as soon as possible. Hope that helps thanks