Best antibiotic for a spider bite infection that was not treated with dicloxicillin?

Unlikely: To have been a spider bite. Most of these (unless you have actually seen and identified the spider as a black widow or brown recluse), are community acquired infection with methicillin-resistant staph aureus (mrsa). Many of these are sensitive to Bactrim (sulfamethoxazole and trimethoprim) or Doxycycline or sometimes clindamycin. Dicloxacillin is unlikely to be effective.