If she needs to talk again, you could tell her that your voice is unusually high because of a medical problem you'd rather not talk about. That way, you'd be telling the truth yet maintaining your privacy. I used to know a guy whose voice sounded like it had never changed (sounded female, basically). In retrospect, I recognize that he might have been ftm, but at the time, I just assumed it was due to an injury or medical problem that would be too personal to talk about. Our other friends seemed to think the same thing.