What Causes Rash On Butt Cheeks?

I got a very bad rash on my butt cheeks that burned and hurt when I was sitting. I took a Allergy pill and it took some of the pain away, but now, two days later the rash is almost over my whole body and looks like a sunburn and feels very rough. On close inspection, there are tiny bumps making up the rash. What could this be?