What happened to you this time is quite common. My guess is that the needle was left in place just a bit longer than usual after they stopped the bloodletting. Any time normal blood isnt moving, it will clot.
If the area where the clot formed is a red and tender, it still isnt dangerous. Doctors call the problem superficial phlebitis. It is treated with warm compresses. You can also take ibuprofen (Motrin, Advil and many generics) or naproxen (Naprosyn, Aleve, generic versions) to help ease the discomfort.
From what youve told me, I doubt that you have a condition in which your blood tends to form clots too easily. So, this event does not increase your risk of major blood clots or stroke.
As you know, though, you need to have regular bloodletting. Hemochromatosis is a condition in which people have too much iron in their bodies. Much of that mineral is stored in red blood cells. Bloodletting removes the red blood cells and excess iron. This is very important because the iron can build up to toxic levels in several organs.
Excess iron in the liver can lead to cirrhosis and to liver cancer. In the pancreas it can cause diabetes, and in the heart, congestive heart failure. So dont stop the bloodletting because of the clot that you saw.