What is this round red shaped rash that suddenly appeared on the back of my thigh?

I'm a 21 year old female and here's what happened. I woke up a few days ago and felt something itching me on the back of my thigh. At first, I thought it was a normal bite. Later, I checked it on the mirror to find a circle shaped red rash. What could it be? I never got anything like that before. I could post photos if needed.

If you have been outside among plants, it could be poison ivy or poison oak. Put Calamine lotion on it and wash your hands well after touching it. Keep your hands out of your eyes.

If You have been outside sitting on the ground, chiggers are also a possibility.

Treating chiggers:
http://boyslife.org/outdoors/wilderness-first-aid-qa/29247/how-do-you-treat-chigger-bites/

Anti-histamines like Benadryl help hold down the itching for both the poison ivy and chiggers.

If you have not been outside, do a search for eczema, psoriasis, contact dermatitis, seborrheic dermatitis, rosacea, and hives.

If its like a ring where it is just red on the out side of the circle and nothing in the middle of the circle it could be ring worm. I had that once on the back of my arm it also itched and took forever to disappear.