I just wanted to add that she has been around a variety of people since I got her at 9 weeks. If anything she has always been overeager to greet people!
We regularly visit Rural King, Tractor Supply and a few pet stores.
I have had her in training since she was 12 weeks old. Often complete strangers will perform the 'stand for exam' practice in our class and she has never, ever reacted negatively.
It just was totally out of character for her to growl.