What Causes Itchy, Red Rashes On Chest And Neck?

Chest rash that is itchy and red, creeps up neck when worsens and gets redder, enhances stretch marks...worsens with hot water, necklace, heat, and unknown causes...started nearly a year ago and has worsened over time. Used to be sporadic, now is constant, but varies in severity...there are a couple of red dots, but no major bumpiness..