Something similar happened to me a few years ago when I was riding a lot training for an aids ride. All of a sudden, when I took my bike shorts off I would have red welts -- though in my case they were all over my butt and thighs. Didn't hurt, weren't uncomfortable. Hadn't gotten new shorts. Hadn't changed laundry detergent. Just a mystery.

I handled it by taking off my bike shorts as soon as I returned home and got in the shower as soon as possible. The problem seemed to subside when the weather got cooler and hasn't happened since.