HELP!

So yesterday, for the third time in the past year, I got a rash on my boob. They all look relatively the same. Normally I get the on the cleavage area, but yesterday I got one on the side/underneath area of the boob. I usually put cortisone on it to relive the itchiness, and it will mostly go away over night. Has anyone else gotten this or know what it is??? Please help ladies!