Why do the backside of my legs get red and itchy but the front side of them stays completely normal?

Forget about the front the real question is why there's an itchy red rash on the back of your legs. Is it always there? If not, when does it occur and for how long? When was the first time? What were you doing the day before, an hour before. What are you wearing? Is it one confluent rash or just in patches. Is it only behind your knees. Answer these questions and you'll probably figure it out.