I have a visable vein on my leg?

I have a visable vein on the inside of my left leg that is about 15cm long starting just below the knee and going about half way down and i have had it forever. I don't think it is a varicose vein because it doesn't bulge and i was born with it. I was just wondering if anyone had any ideas why it is visable and why it causes my leg to ache around that area sometimes.