Duffy, for clarification she's 5 1/2 YEARS? Not months? No, this is NOT "Normal" !
If it's YEARS, then we'd need more context: when it started, your response, etc. "Nipping" is a euphemism for biting, which needs to be corrected, this is unacceptable and will likely get worse the longer you let it continue.
Again, assuming it's YEARS, a quick tug on the leash as she lunges, with a "NOOOOO!" is in order. If that doesn't work after a few encounters, have the vet take a look, it's really unusual for an adult to develop this type of reaction, either she doesn't feel well or is developing an unrelated anxiety disorder which might need meds.(SSRI'S).