reoccuring blister on wrist

Every couple of months I get one or two blisters on my inner wrist; they have a white center filled with pus and are red around the outside of the center. They start itchy and soft and grow very large and hard. They last about 5 days before going away on their own. They are very painful and cause my whole arm to the elbow to swell and I get a red streak up my arm leading from the blister to the elbow. My muscles get very tight and I have knots in them from day one through about day 4. I have had a nurse look at them and she just popped it and covered it. I have used cortizone cream, covered it, left it uncovered, hot soaks, ice soaks and pain reliever for the pain. The doctor didn't have a clue. Does anyone know what this could be?