I notice the veins in my chest and body more when I'm rolling, too! I think it's more a visual effect from the drugs, rather than an actual changing of how noticeable they are...as well, when your body temperature increases as it does with mdma, your blood vessels (such as veins, which hold the highest volume of blood out of any vessel) will raise to get closer to the surface of your skin, where they can release some of the heat into the air and be cooled by being closer to the cool air. The veins in my hands are more prominent when I'm hot, and that includes when I roll- it doesn't happen to everyone but it's definitely normal!