This condition can be caused by the bite of the brown recluse spider.
Usually there is an area around the bite site where small, circular
infectious looking lesions occur and these can spread as the condition
worsens.  To see a vet is your best option, as cortisone is the
recommended treatment.  Cortisone can also inhibit healing if the
cause is an infection, though, so a vet's opinion should be sought
here.  I hope this helps you.
 
Cordially,
Greta Hoisington
[Posted in FML issue 4166]