BetwixtBetween wrote:
You know that stereotype of that woman who takes everything as a comment on her appearance and especially as a comment on her weight? Those women exist.
She probably thought you were mocking her weight. It's probably a sensitive subject for her if she thought she'd find something that would fit her in the plus size section. You probably added to her embarrassment when you repeated yourself, since, if nobody had heard you drawing attention before, they'd probably heard you the second time, looked, and saw her.
So yes, in her mind, and in the minds of many other girls, you said something wrong. I'm not sure how you can fix it beyond saying something to one of your mutual friends like "I was really surprised to see Susy head over there. She's definitely not plus-sized." Maybe just leave it at that. Pick someone she consults with. With any hope they'll pass along your side of the story.
This is the most likely problem and also has what will probably be the most helpful fix.
To say "you are in the plus size section" without some qualifier that you think it's accidental will be taken as a cruel comment. To an NT it doesn't sound like "you must be in the wrong section by accident". It sounds like "you must have recently gained weight and you hid it well but now I know bwhahahahaha". Repeating it (but still with no qualifier that you assumed it was an accident) just makes it sound even crueler.
Hopefully it can be fixed the way BetwixtBetween advises.