Masterbating while lying down. mid session, heart began to pound then had shallow beats and strong palpitations followed by bad heart burn. ?

Heartburn: The activity caused increased adrenaline along with the heartburn to trigger irregular heart beats. See your doctor if it returns.

Conditioning: From your description is appears that you are not in a good physical condition and need to exercise regularly. Masturbation per se is not harmful. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco or alcohol in any form. Practice safe sex.