I am underweight and have spider veins?

Hereditary factors.: Spider veins are very common. Most likely there is a hereditary component. Hormonal changes may also play a role. For now the best treatment would be staying physically fit and physically active. If the look of the veins bother you, then sclerotherapy could be considered. Most likely you will develop more vein with time.Spider veins will not cause you harm but they can be treated.

Spider veins: most commonly hormonal related also there are people that spontaneously bruise secondary to weakness of the blood vessel wall which can lead to spider vein development as the body increase local blood flow to heal, there is literature supporting adding bioflavonoids to diet to strengthen blood vessel walls