What Causes A Rash Between The Chest And Armpit?

I have had a rash on my upper inside arms and chest. Taking steroids now , second time. It activates when I go to the pool and seems to be most angry with moisture and sweat. It s itchy and annoying. No explanation, so may have to go to an allergist unless you have seen similar before, This is an ongoing rash and nothing seems to clear it up.