It looks like veins. And, if it's coming back in the same pattern, same spot, that makes it more likely. But, I'm not an expert, just guessing. I get something similar on my legs when I get cold, or sometimes when I get cold and then warm up quickly. It could be normal for you, but if it's really bothering you or you have other symptoms along with it like swelling, pain, fever or bruising, I'd have it checked out.
That is really odd. This is a heart rythm community though, so I doubt you will get much help here. You should post this in general health or undiagnosed. Post back here though when you find out what it is. Interesting.