I disagree with h0ppy and agree with doolange - it's B vitamins, but that's assuming you're taking some sort of supplement. Or it could be that you're reallllly dehydrated (makes urine darker though not usually a bright color) or possibly you ate something like asparagus that helped your kidneys clean house.
But normally it's B vits, and that's just how it works - nothing to be concerned with.