What To Do For The Rash On Both The Arms?

good morning!
i have developed a rash on both of my arms.. not in the pits but underneath my arms where it meats my clothing. this is the 2nd time this has occurred.. the first time the rash was blotchy & red almost bruised like.. this time the rashing was in larger sections almost like welts-- raised and extremely irritating. I did apply a cortizone cream yesterday, but so far it has not made a difference.