Why do veins (muscle fascia?) spasm?

Every so many days/weeks in notice a vein/artery in my leg thats seems to be spasming, sometimes in my arm, too. I guess it could be a muscle fascia (sp?) but it seems to pulse with a kind of heart beat regularity. What is that? It’s kinda gross. :confused:

It happens to me too. It feels like there are air bubbles or something like that in my vein. I’ve had it in my arms, though, too.