Luz has always had this carryover of feelings from when they met, before Amity arrived. Luz was starting to kind of quietly think about it, and then Amity came.
So her instinct was to sort of brush it off, thinking that she'd be able to stay JUST friend with Aubrey. But then she learned Aubrey was with Eren and failed to fool Aubrey that she was fine with it.
And through that, well. Now Luz knows its possible to care about two people the same in different ways, and knows that this could also just destroy her whole home life.
It happened unexpectedly for her too. She never thought SHE would feel that way about two people. But she knows now that its possible. She is emotionally invested in both of them.