Sunburn Looking Rash On The Hip, Buttock And Stomach. No Itching. What Is This?

I developed a strange sunburn looking rash today. I have not been sick or taking any medications . I had to make a two hour drive today and when I got to my destination I used the restroom and noticed a slightly bumpy sunburn looking rash on my let hip, left buttock, and stomach . I also notices it on the left side of my neck . It s not sore or itchy, but it is very warm to the touch. I have not changed soaps or detergents. Another strange thing, the rash skipped over my underwear just like if I was laying in the sun and stopped at the waistband of my jeans. Any thoughts?