Why are veins in my shin getting swollen and sore?

The veins on my legs are quite prominent, but my doctor has always said they are not bad enough to need treating. Recently, though, one on the front of my shin became swollen, hard and sore, and my doctor told me the vein was inflamed. I was given anti-inflammatory tablets, and it does appear to be slowly getting better. But why has it happened? I haven't injured my leg.

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This sounds like superficial thrombophlebitis, where a vein near the surface of the skin becomes inflamed, which makes it red and sore.