What Causes Red Rashes On Chest, Back And Neck?

Hello, I occasionally get a red speckled rash that starts on my chest, back & sides of my neck. I started with the same rash yesterday and it is worse than I've ever had. The rash is crawling down my face, scalp, arms and back. How can I figure out what I'm allergic too and does this rash seem like an allergic reaction? I'm a 39 year old female, 5ft 2in, 115lbs.