Have this weird line on my right leg, goes around the circumference of my thigh. All the at around and connects. Doesn't hurt. Is darker then my actual leg. About a quarter of an inch in diameter. What would it be? Or what would cause this. I've had it as long as I can remember. It blends in with the rest of my skin. It's not raised or sunken In. It was much darker when I was a child. It's lightened up threw the years while my other birthmarks have stayed the same color.