Rash on back of knee

Any idea what this may be from? It's been there for a few days and it was super light at first and it wasn't really a circle but now it is a circle and got darker. That's the only place I have it and my other leg touches it all the time so it isn't spreading. It's not scaly, just red and a little itchy but it only bothers me at night. It's hard to see in the picture because it's the back of my knee, but it's the same color throughout, so I don't really think it's ringworm 

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