What Causes Bulging And Sinking Veins On Arms?

I don t have any health problems that I m aware of. Im a 19 year old male, I weigh about 125, and am pretty skinny. When my arms are lifted upwards my veins seem to sink in. When they are resting at my side they bulge. And it s not just a few veins, it s many. I notice they bulge the most when I play guitar. I m 100% drug free, never tried anything, I smoked for about a year and a half, but have recently quit and replaced my cigarette smoking with chocolate milk drinking, lol. What causes this? Is it normal? I don t necessarily feel pain cause of them. I notice in a lot of guitar magazines, guitar players arms have bulgy veins. But I never hear anything about sinking veins. I can watch one vein, when my arm is raised, it s sunkened, when it s lowered the same vein expands. I m pretty spooked out about this. I notice every now and again veins in my legs bulging. Those ones can be painful