What Causes Calf Pain While Walking?

So yesterday afternoon all the sudden I noticed that I had this pain in my calf that is only there when I walk, the pain is not that bad it just sort of feels like a slight cramp in my calf when I walk on it. And me being the worry wart that I am automatically think blood clot....I can sort of see the vein in one spot on my calf and im just not sure if I am being over paranoid or not?