Thanks for the additional information. One of the possibilities is a condition called pityriasis rosea.
Pityriasis rosea usually involves the following: it begins generally with something called a 'herald' patch (kind of announcing what is to come, if you will), a round/oval pink patch 3-6 cm in diameter (was the first one on your knee larger like this? If so, this is diagnostic for pityriasis). This is followed 1-2 weeks later with pinkish oval lesions, up to 100 or so in number, about 4-5mm in diameter. These lesions often have a ring of scale around their border, and can actually form a "christmas tree" pattern on the trunk, with the base of the tree on the waist. This usually resolves spontaneously, on its own within 1-2 months. The exact cause of this is unknown (some think that its related to a virus, others to exposure to certain medications). 1 of 4 patients (25%) can experience fever, headache, sore throat, fatigue, but back pain is not related to the disease.
In general, pityriasis is not treated, as it resolves on its own.
Your rash doesn't sound like a lot of other entities. It doesn't sound like dermatitis (eczema), nummular eczema, or infection with rocky mountain spotted fever, scabies, or other skin disorders. Nevertheless, the possibility still exists of a different diagnosis, and perhaps something infectious. One thing I couldn't rule out from your description is the possibility of secondary syphilis. Now, it actually turns out the this disorder is on the rise in certain parts of the country. It is a sexually transmitted disease, and so it is a concern. Syphilis can often involve (first) a painless, open genital ulcer. The rash of syphillis often, however, involves the palms and soles, and the lesions can coalesce into larger structures (this doesn't match your description). It can, if not treated, over time have considerable, long-term central nervous system effects, among others.
So while I think pityriasis is most likely, and if so it will resolve on its own, you might want to have your primary care provider take a look at it. Better safe than sorry.
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