I am having the same problem but it occurs on me on the undersides of my arms. I am using Coppertone Sport SPF 50. I thought about it and noticed that for some reason it is where I apply heaver layers of sunscreen than on the tops of my arms. I read where this is heat rash or sometimes called Prickly heat and can be caused buy the pores of the skin getting blocked and not being able to sweat naturally. It has similar symptoms on me to Poison Ivy and I have been treating it in the same manner. One thing I have been doing is only using a very light amount of sunscreen on my arms and after my rides I wipe them down with paper towels before showering. This has helped reduce the irritation and I have noticed the sores are starting to go down so for now, so far so good. Hope this helps.....