I am a 19 year old girl and I have very visible blue leg veins. I have them for about two years. One vein in the back of my knee bulges out a bit but the rest of my leg veins (quite a few) do not. Also i am not overweight, don't stand or sit for long periods of time, none of my parents have varicose veins and I do not use birthcontrole, so there is almost no chance it are varicose veins. But what can it be?