Have you worked out your hands within the last few hours? I play keyboard/piano (plus I use a computer all day for a living) and after I have been using my hands like that for a while, the veins in my hands dilate because of the extra blood flow that is necessary to provide the oxygen to the muscles. If there isn't any other symptom it might be that. If there are other/unexplainable symptoms, I would talk to a doctor.