Does she live with another pig?
I'd start with the assumption that it's a fungus, and treat for that. Get some Nystatin or Lotrimin and apply sparingly twice a day with a q-tip, rubbing it in well. Do this for a week or so and see what happens. If the bald spot gets larger, the switch to a triple antibiotic cream and treat the same way.
We start with antifungal medicines first because antibiotics can sometimes make a fungal infection worse, whereas a fungal medication won't do anything to a bacterial infection.