At first glance, looks like some kind of viral infection..can you please elaborate more like:
When did these rash appear..recently or its been many weeks..did you notice any single initial rash before you had these rash.., are you on any medications, did you eat any different meal recently or been somewhere..?? It could be any urticaria, or probably pityriasis rosacea, but the upper part of your back somewhat resemble pityriasis versicolor. But the time of onset could help exclude some diseases.