It would help to know a little more about the rash. Is it lacy, bumpy, scaly, blistered?
I had a poison ivy rash last year for my first time. It started out with little bumps that looked like bug bites (raised bumps, reddish skin). After a few days it blistered. I had no pain other than itching. Calamine lotion helped, as did cool compresses. It took about a month for the rash to heal. I read that a person's first posion ivy rash can take up to 3 weeks to show after the exposure to the poison ivy plant.
I always thought chiggers are more itchy than painful. I am wondering if you have a heat rash, yeast rash or just general irritation from something rubbing on that area. With the warm, humid weather those may be a possibility. Is it possbily sunburn - maybe you wore a lose fitting shirt or tank top outside?