Your description suggests that you had bleeding into the muscle of the arm. This commonly indicates that the needle that was placed into the fistula may have gone through the fistula vein and entered into a muscle around the fistula. When the needle was removed, blood leaked from the fistula into the muscle and caused significant swelling and often causes significant pain as the muscle swells. It may take several weeks before the blood is dissolved and the swelling in the arm resolves. This is an unfortunate event, and thankfully this does not occur often. Great care must be used when placing the needle into the fistula so as to just enter the vessel and not traumatize surrounding muscle and tendons.