What Causes Itchy Rash On Back Area, Arms And Legs?

For the last few months I have had a rash over my back, arms, and legs. There is tremendous itching. I went to my family doctor and he changed my blood pressure medication with the thought I was having a reaction to it. This did not help. There are no changes in soaps of any kind. I have been told to go to a dermatologist. My appt is in March. Any ideas.