Probably when the possibility of headlining a UFC card came up. Far as I know, she doesn't have anything booked at the moment. Only logical she might defend her belt again.
Seriously, some of you guys are acting like she's going the Conor route in not defending her belt at all. She has three defenses on her Invicta belt, and she dispatches every single contender with ease. Some of you are acting like the fights were a fluke, like a 13-second KO over a dominant champion and that she's been ducking a rematch that she might very well lose, like Conor with Aldo.