There is a tiny knot on top of my hand that seems to be in my vein. I especially see and feel it when I ball up my fist. Could it be a blood clot?

Yes. Phlebitis in that area is a possibility. Probable due to trauma as long as there is no redness or extension there is no reason for concern. Take Motrin and put a heating pad when idle and should resolve in a few days.
Unlikely. Although small clots can and occur from trauma. More likely you are encountering a valve in the vein in your hand.
Nonsense. Valves in veins are very delicate and flimsy structures, i would like to hear from a reputable phlebologist or vascular surgeon that he can feel a valve in a a vein.