I Have This Itchy, Red Bumpy Rash On My Butt

I have this itchy, red bumpy rash on my butt and back of my thighs. I've had it almost a week. Last Tuesday I unloaded a truck at work in the cold and towards the end, I felt like my butt was on fire, like I smothered Icy Hot on it. A couple days later I noticed the rash on my butt when I took a hot bath. The heat seems to make it itch worse. And in the past few days, it seems to be getting worse and has spread from my butt down to the back of my thighs as well. I've tried benedryl n hydrocortisone cream but nothing is helping. Any ideas what this could be?