What Causes Itching Rash On Chest Between Breasts?

Hi, I am a 41 year old female. At Christmas time I noticed a small patchy melasma type rash on my chest between my breasts. Over the past few months the rash has spread down my chest over my upper abdomen. It is a bit itchy and I have no idea what the rash could be. I tried putting a corisone cream on the rash to stop the itching but the rash seemed to spread. Any ideas?