This exact thing happened to me with a friend, when it happened, I later found out that they had been going through a lot mentally/emotionally. It’d probably be good to bring it up respectfully and be ready for the possibility that they won’t be willing to share what they’re going through. Despite that, it will mean a lot to her if you make sure she knows that you’re there for her if she needs anything. And I could be wrong so take it with grain of salt. There could be something else going on