What Causes Painful Blisters Post Cortisone Shot?

Had a cortisone shot in the bottom of my foot in May. Now, the area of the injection is red, painful and feels like a bristle brush is rubbing against it when I walk. It s getting more and more painful by the day, that I m afraid i won t be able to put any weight on my foot in a few days. What is causing this weird condition?? Can t be shingles, I had that about 15 years ago.