Small movable lump midway up penis shaft under vein. only felt when erect. rarely causes pain. hasnt grown. is this serious? will it affect my vein?

Need to examine: It is not possible to be sure without examining you, however, what you described is consistent with a sebaceous cyst. It will not cause any harm, but may grow gradually. As and when you can, you should get it removed. 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, alcohol, weed or street drugs in any form. Practice safe sex.