I think the first outfit was the BTW era because thats where she stop performing, so it was kind of obvious that she has to return where she left, she has to finish that moment: The BTW ball.
This was performance was supposed to be the final show of the BTWB since the show when it was canceled was the 99th. So that is why I think She didn't have a btw era, but I also think it was the nun's outfit.