Might be lupus.
That must be it.
There could be a chance that you have muscle loss so every time you open your arm, your skin begins to press down on your veins or, generally speaking, your skin is on too tight.
In all seriousness, get it checked out. There actually might be a chance of muscle loss.
My muscle count is okay, I'm exercising daily.
Anyways, I whole stuffload of people on the internet had this problem. They said it went away, I might as well do the same.