How does skin necrosis from a spider bite takes place? Will it happen from the inside of the skin out?

About 4 or five days ago I noticed a large red pimple on my right thigh that frlt hot to the touch and it has been growing since. Have a little fever that always starts late in the afternoon through midnight. 2 days ago I went to the hosp but they didn’t think much of it and prescribed a wide-spectrum antibiotic and Ibuprofen. Today, early evening, I has REALLY grown, very hot to the touch, still feverish, headache, rash on both legs and area closest to the center dot of inflammation is starting to turn very dark red specially under the skin. So, is this how skin necrosis starts, from the inside out? Answer appreciated.

Minnie