Here i can just assum according to your saying
that is more than likely not a vein you are seeing may be brachial artery
. this could be a couple of things actually that i can think of off the top of my head(i am sure there are others too) but it could be that that particular vein is sitting on top of that brachial and making it "appear" as its the vein that is actually throbbing. OR, if it is pulsating more it could be a higher than normal BP going on?
Monitor the BP once any abnormal BP records, then consult your physician agains