What Causes Itchy Rashes Behind The Knees Of Both The Legs?

I have had a persistent rash on bothe legs just above the knee on the inner portion of my legs and behind the knees. it has a burning sensation, itchy and the affected area become clary.. I have to wear PJ pants to bed because if my legs touch they kind of stick together and it hurts when I seperate them. can you give me any idea what it could be? I have tried allery medication and it doesn t help. this has been going on since before xmas