What Is The Remedy For Tiny Blisters In Knee?

sir,i developed a blister on my knee more than a week back.It was highly painful so i burst it.After that it developed a blackish color and a ring of red around the central wound.Afraid that it might be gangrene i consulted a GP who advised me droxyl 250 and antihistamines.He said that i have been bitten by a poisonous insect which has resulted in a severe allergic reaction.I have been using the medicines for the past 3 days but instead of abating the expanse of the red ring has increased.Moreover,more small blisters have developed around the central wound on the red ring.The diameter is about 4 inches.What should i do?