I had something similar happen recently so hopefully this will help a little bit. I was doing something where I had all my weight bearing on my right arm. The next day my arm was sore and a vein on my inner elbow (I think it is the median cubital vein) was very prominent and bulgy for about an inch or so. My veins are never noticeable there! I freaked out so bad that I had a clot somewhere in my arm and made several visits to different dr.'s, all who told me my arm would be red or blue, painful, swollen, etc. Of course I wasn't satisfied because google told me that symptoms don't have to be that obvious
So in one dramatic trip to the E.R., they did an ultrasound of my arm and all was fine. No clots. They never really explained why the vein popped out (blood draw a a few weeks earlier maybe?), but I'm not worried anymore. All the dr.'s told me that a blood clot on the surface will feel hard like a solid lump or an electrical cord under your skin(not spongy/soft like a regular vein), and it will be warm and probably red. Also if it is a surface vein, they are harmless anyway and will go away on their own, they will not move to other parts of your body. I'm not sure how close my situation is to yours, but I hope that helps a little. Also, google this and see if it's close to your situation: "healthy berg" "my not so broken blood vessel". Again, not sure if this is exactly what you are talking about, but if it is it's very reassuring because this girl had it happen while working out to and it turned out to be nothing that just eventually went away. I know googling isn't good normally, but this one is reassuring.