Zerging is really just a matter of DPS. When your girlfriend hits a mob with stunning blow and kills it on the followup swing or crits/kills a mob in one swing it is zerging on a micro-scale. When she kills a mob with every other swing from the start of the quest to the end of the quest and the bodies are like bread crumbs from a children's story, then whether you like it or not, you have zerged.
Zerging has a bad name because people who can't zerg try to zerg and get the party or themselves killed. Run a non-epic quest with a 70+ strength 800 hp barbarian who is geared for epics and you will see what zerging is meant to look like. Its sick, awesome, but sick.
The moral of the story is: If you are having fun, then you win. Its ok to be level 14 for the next decade if you are having fun doing it.