What Causes Itchy Skin Rash All Over Body?

hello. i have developed an itchy skin rash on various parts of my body and my doctors cannot figure out what is causing this rash. The rash started in late february and cleared up with use of topical cream but has flared up again with a vengence to now include my scalp. I have stopped taking all vitamins/supplements for over 2 weeks now but the rash persists even with the use of persciption topical cream. your thoughts?