I was bit by a spider on the thigh a month ago. It was red and infected with a bruised ring around it. The doctor ruled out a tick or widow and said recluse bites were common in that area where I was vacationing.

I left it for a few days to see if it'd heal on its own before I went to the local clinic. They gave me a week of doxycycline. I took a couple of the pills but felt like garbage. It cleared itself up without any more meds. Recluse bites, I've learned, usually don't turn necrotic.

You could call your doctor for some blood tests etc to be safe.