What Causes Bruising Along With Itching At The Point Of Blood Withdrawal?

It has been more than a month since I donated blood, something I have done countless times without complication. This time I received a sizeable bruise which turned a very dark black/purple. It took over a week for it to begin to fade and then it became itchy...as the bruise faded it seemed to be replaced with a rash. I thought at first that this was a case of itching while healing occured, but the bruise is long gone and the rash will not go away! It is larger than a dollar bill and even when I am not scratching it it is pink with raised bumps and slightly warm. To me it looks like exzema. Is there any explanation why a bruise would develope into a rash?....and better still any cure for the itchiness?