Suddenly my chest and arm veins are very visible?

Not concerning. The prominence of veins through the skin is affected by many factors including levels of hydration and dehydration, body temperature as well as outside temperature, medications and even levels of stress. Fair skinned people tend to have more easily visible veins than darker individuals and hereditary plays a strong role also. In a healthy person, visible veins are usually not of any concern.
Low body fat? . Sometimes when people lose a lot of weight or body fat, their veins show more. Sometimes with weight lifting or steroid use? Has something changed for you?