What you are describing could be necrotizing fasciitis, and although MRSA is a common cause there are many classes of pathogens responsible. Any of them can constitute a medical emergency, and if not diagnosed and treated quickly a surgeon may need to butcher you to save your life. I've been in the OR for a debridement and you do not want to be that patient.

If you have pain way out of proportion and darkening skin (or any rash for that matter), that's not the time to play House. Call your doctor if you have one, or just go directly to the ER.