One time my dog was barking at the front door because my husband's nephew was there. He was not familiar to my dog because he had moved away and was just in town to visit. I walked up to get my dog to put him outside to let the nephew in. I didn't say anything, just reached down and tapped his back end. He turned around and bit. He got my hand. As soon as he looked up and saw it was me, his ears went back and he started hunkering down. He felt so bad. His eyes were so huge. I, too, was upset but after realizing that he had never showed any aggression towards us before and he had no idea who was touching him from behind and there was a stranger at the front door, he was just scared and startled. He had never showed any aggression towards us after that either. Under the circumstances you described, I wouldn't worry about this one incident.