I think we should give this elderly lady a break. She was likely not raised in a co-ed environment, she likely knows nothing about the TG community, and Bree's presence in the changing room took her by surprise.
Bree, I spoke to my SO about your predicament and she suggested that the next time you are asked, "Are you a man?" in a ladies changing room, the best possible way to respond is to have the presence of mind to say, "I'm transgender. If this bothers you I can leave until you're finished using these rooms." This would then allow the person to either say, "Oh, that's all right", or ask you questions if she didn't know what "transgender" is, as alternatives to having her run back to her cubicle, frightened.