If it is red..take a pen and draw around the redness. If after time the redness spreads outside the pen markings/circle...see a doc. I waited too late and it got pretty serious. It ended up eating through everything down to the bone and a year later it has healed but the leg has a permanent dent and is still black. Don't wait if you're sure it's a spider bite. They are also know to cause staph due to the wound being a puncture wound.