I think the best step to take in this situation is to talk to her about what's going on. She's the only one who knows what she's feeling. You can speculate all you want, but I believe you'd feel a lot better with answers, and she's the only one who can provide them. So talk to her, let her know how you're feeling and that you're concerned. Ask her if there's anything you can do to be there for her. Let her know you care for her and support her. Hopefully she'll be able to open up to you if you come from a place of care and concern.