I don’t know. Maybe she is struggling with self-love, self-compassion, or maybe she is feeling down about herself in general. It is also possible that she is going through something right now in her life – a life change, situation, family situation.
The only way to know for sure is to ask her. Something like “Know that I am here for you no matter what, but I have noticed you seem more down than usual and taking things in a negative light more than usual. Is everything okay?”
Don’t push her to answer but ask with love, genuine concern, and do it casually. If she tells you, then maybe you can help or help her find help (like a therapist). If she doesn’t, then give her some time. She might come to you on her own in time, but you could mention it every so often.