No, she isn't growling to protect you. As others have said, it is frustration and redirecting on you. It can go from the snapping at you that occurred to actual bites. A group class for OB and I would suggest a private class as well to work on the handling and management.
I can say from experience (and the battle scars) you seriously want to get a handle on the redirecting before the snaps turn to bites. My troublemaker - different issues - skipped the snapping and went straight for the bites. It ended only after a 'come to jesus' meeting.
Keep in mind for your dog's and other's safety, that dogs who redirect can redirect on whatever is closest to them at that moment. This time, the snap was focused on you, next time it could be a child, neighbor, family member.