i can't see it as you know nor can i take a more detailed medical history or examine you to narrow down the possibilities. this can be a bite (spider? ) or you may have secondary infection. just a blind guess. without a hands-on examination you cannot be properly diagnosed or treated so i suggest that you seek a medical evaluation today.
red burning itching circular bite on dorsal thigh. now is a 10 inch red circle. increasing redness and itching and burning from a central bite on the back of the thigh. began as small red bump - now is a 10 inch red swollen circle on back of thigh.