What Could Be the Cause of my Hives?

Two weeks after having surgery on my broken ankle where screws and a plate were put in, I came down with a severe case of hives almost all over my body-- also had swollen lips and chin--now have swollen eyes. Have had the hives now for four days. Each day some hives go away and new ones show up in different places. Have not taken any pain medicine since the operation-just two adult aspirins (blood thinners) which I stopped at the onset of hives. Never been allergic to anything in my 65 years.