What is the cause of veins popping out on my feet? it's also started popping out around my breast. what could it be from?

In the feet : When you stand the veins get engorged in blood and this is why they seem or appear prominent. There is not a lot of thick skin on the top of he foot say as on the bottom. So the veins become more visible.

Varicose : Veins are common in the foot and ankle due to long periods of standing and gravity pooling the blood. I can't speak to why in breast.

Multiple reasons: Veins return blood back to the heart and can carry a large volume of blood. Depending on many factors including levels of hydration, genetics, outside temperature and even physical activity, the size of the veins will vary. Another important function of the veins is to help control body temperature and this too will affect vein size. Also with aging, veins enlarge as tissue volume decreases.