I suffer from pitting edema and I notice that when I have to stand all day I get little warm red bumps on the lower part of my leg near my ankle. Why?

Depends. A lot of times with venous insufficiency you will see a venous stasis dermatitis develop. Would need to have evaluation as to why you are having this edema problem.
Veins . Usually due to venous engorgement. You need t get support hose.
See below. Agree with Dr. Davis. Try reducing your salt intake to see if that helps.
For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. 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, if you have sex.