How To Treat Rash And Itching On Thighs?

I have had a rash and itching on my both thigh for a months. It started out small and had become larger and now is starting to spread in my lower abdomen and around umbilicus.Now it started to spread to other parts of also. It have a round shape but the edges are a little raised and little fluid filled. I already went to my doctor and he gave me a allergic medicine .The medicine is not working properly Please help before it gets worse.I am 30 years female.