What Causes A Red Mark On The Thigh?

I have a red mark the size of a quarter on my thigh. It has been there for a while and while it looks like one big hive, the skin is not raised and it does not itch. The skin is smooth and it does not have dry or scaly patches on it. It does not hurt and if the skin is pulled taught, the color goes away. I have no other symptoms or problems. Is this some sort of allergic reaction?