I'm A Female Who Is Almost 18. I Have Been

I'm a female who is almost 18. I have been taking oral birth control for over a year. My breasts are semi veiny. And sometimes they are more veiny than others. When I am inactive they show only one vein that runs through my nipple. But if I shower my veins are very noticeable. The veins in my body aren't as noticeable around my body but the in the same circumstances my other veins will stand out as well. Why are my boobs more veiny relative to the rest of my body?