What Causes Swelling And Itching In My Leg Below The Knee?

I'm 29 and I woke up three days ago with a pain in my leg. I looked and it was the vein just to the left of my right knee which was always quite prominent . It looked swollen and it was and still is hurting alot. Now 3 days later the skin over it has become redish and sort of itchy but only when I touch it. The area covers from my knee three quarters the way down to my ankle bone.