sometimes when you have blood drawn, some blood leaks out under the surface of the skin, it hardens and forms a clot. it will go away, the body will naturally take care of it. you had said it was a larger than usual needle, perhaps that caused a bit of leakage. sometimes it appears as bruising (usually a new nurse who doesnt know how to stick you properly), but if it was just a small local leak, you would notice more of a bump than anything else. of course, always consult with a physician when dealing with your health concerns.
an allergic reaction to rubbing alcohol
if its a small lump say the size os a cough drop or smaller probly is what has ben stated before if it is large you need to have it checked out as soon as you can cause it might be a distended vain happened to a coworker after donateing (he didn't take it easy used the arm excesivly unloading a truck ) and plese take care if you sea redness around the sight and or going up down your arm get to the emergency hospital quickly you might have thrown a clot in the vain wost case death or loss of limb take care
bruise or perhaps lymph node? Best to go to your GP.