Just diagnosed w/cellulitis (spider bite) current hepc tx weakened my immunity. How serious is this?

Possibly. Depending on the severity of the cellulitis, it may possible to continue treating your hepatitis c while also being treated for the cellulitis. You should notify your your treating physician if you haven't done so.
Serious. Cellulitis diagnosed in a person who has just finished treatment for hep c which includes interferon are generally immunesupressed. This is serious and need to be started on antibiotics, and may need intravenous antibiotics as soon as possible depending on the severity of infection. So you need to be reevaluated very soon. Even low grade fever is considered serious in these situations.
Brown Recluse. We are seeing an increase in brown recluse spider bites. These can progress to significant tissue death and with hep c worsening morbidity. I would present to er or local hospital based wound clinic. The area should be elevated with possible cooling to prevent progressive tissue death. Antibiotic coverage should be initiated immediately prior to developing worsening cellulitis.