She probably knows about hair extensions as most girls use them these days.
She probably & quite naturally, thought maybe you'd had hair extensions.
When girls hair looks thicker & better, its because they've used extensions.

If you think about it, it was a perfectly innocent question.

The point is, she didn't really care and wasn't embarrassed about asking like she would have been if it had looked bad or obvious.
Its when people stare and say nothing at all that its a sign they suspect you're wearing a piece.

You have to think about how you feel about it and if it makes you think you look better.
It sounds like the overall consensus was that you look better.
What she said wasn't a negative.
It just disconcerted you because you haven't formed your own opinion yet on how you feel about it.