Aaaawww! That made me cry. Either you look more like a woman than you think, and/or putting out strong femme vibes, the woman is familiar with transgender issues or has a transgender friend.
Whatever it is, that was so kind of her, and wonderful for you!
Since I'm only partially transmale, I dress androgynously, but my general behavior and attitude usually gets me correctly gendered anyway, at least in Thailand.