In her defense, she thought she had a connection with you, and made the daring step of saying hi to a stranger in strange clothing. She was understandably embarrassed when she got shot down for being completely wrong. I'm not saying that getting defensive is the best answer (it isn't), but it is the natural answer for an approximate 100% of the human race.

I doubt she was really trying to push her idea of which tartan was correct as much as she just didn't have a clue how to handle the awkward situation.