I Have A Skin Rash That Has Spread From My

I have a skin rash that has spread from my waist and groin area to my back and chest, very rapidly. I feel like it has become in the last few hours. I saw a dermatologist today and she prescribed a steroid to control an allergic reaction. It is 11 pm and I fear from experience that it will get very active and itchy tonight. I am supposed to start the steroids in the morning, but am wondering if I should the first dose tonight so that we can immediately help control whatever is happening. I also ran out of steroid cream.