I had something similar to that after hurricane charley, I assumed it was just a really bad pimple or mosquito bite at first. Turned into a necrotic staph infection with abscess within 3 days that covered a hand sized area of my left shoulder. First thing the doc asked when he saw it was "that looks like a shotgun wound". They weren't sure of the exact cause, could have been a spider bite, ingrown hair, infection from a small scratch. After the first day it was just really large and swollen, pretty sure if I had gone to the doc then they may have been able to just give me some anti-biotic or drain the abscess and saved me the 2500 dollar bill to operate on the wound. So yes, go now, whatever it is it's easier (and cheaper) to fix now rather than later.