Thanks for supplying this additional detail. While online I cant state with certainty what anything is, I can surely give you my best guess based on what you wrote.
A possibility I am considering is that you may have ringworm. This is not caused by any worms; the fungus that causes ringworm got its name because it leaves round lesions. They are red, itchy, and can leave a crust, but not always. If this is ringworm, simple cases are treatable by over the counter anti fungal ointments- the type you'd use on athletes foot. Clotrimazole, for instance. Fungal rashes take a while to go away with treatment, a few weeks. As ringworm is a fungus, the benadryl and polysporin wont really help. I am happy you didn't try hydrocortisone cream, as if its a fungal rash, rashes on steroids tend to grow.
Another (and to my mind) less likely possibility is that this is the rash of shingles. Shingles is a reactivated form of the chicken pox, and as the virus lives along nerve cells, it will create a rash along the path of a nerve...on one side of the body. The pain here though is usually intense; I mention it because you describe it as "burning" when you scratch it, and nerve pain can be that.
I suggest a visit to your doctor to find out what this rash is; both ringworm and shingles are contagious. And, I cant online tell you which it is; only an in person doctors exam can appropriately diagnose and treat.
Here is more information on ringworm: The pictures on the right side of the article show severe cases; your rash (if ringworm) is not so severe and may look somewhat different. http://www.nlm.nih.gov/medlineplus/ency/article/001439.htm
I hope I was able to help,