Amy seems to have decided to use "Yeah" for affirmative replies.
I wonder whether she picked this up from a film, or where it comes from. The phonetic similarity to German "Ja" presumably plays a role.
I mean, sometimes "Yeah" is appropriate in colloquial English, but it carries a certain stylistic connotation, which I don't think she intends at all.
Unrelatedly, the other day, while talking about rhyming words, I mentioned "need", and she asked me what that meant. I told her, and she said, "No, that's 'meed'." Which is how she always pronounces it. So apparently that's what she thought it was supposed to be? Not sure whether she believed me when I said that no, it's really "need". Ah well.