 [ Symptoms ] 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 [ Disease ] i can not 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