What Causes Itching And Burning Rash Behind Ears?

I realized about five days ago that i started to get a rash behind one of my ears, the next day i had it on both ears and it was pretty bad. I applied cortef but it seemed to get much worse. I do have sensitive skin and am prone to rashes but have never had any behind my ears. Its VERY itchy and it burns. I havent done anything that i can think of to cause it.