Me and cousin are growing distant

Recently, I've felt distant with my cousin who I've seen as my best friend for almost my whole life. Everything was fine, until one random day, she just changed. I don't know how to describe it, but it was like some switch just flipped and she turned into a robot. She used to treat me like a sister, and now she talks to me like I'm her coworker, and it's really been bothering me. I don't expect her to tell me everything about her life if she doesn't want to, or for her to always be happy, or to always want to hang out with me, but all of our interactions stopped feeling natural.

Every now and then, she'll treat me like the cousin that I remember being around, and then the next second she turns back into that AI chatbot personality. It just feels so impersonal and artificial. It might be because she's becoming more of an adult and feels like she should be treating me different because I'm younger.

It's been making me want to hang out with her less, and, frankly, I fear I just simply don't like her anymore. I feel bad, because she's not a bad person and she hasn't really done anything wrong, but I feel like I'm talking to a brick wall. If it's of something she's going through, I wouldn't want to abandon her, but being around her is really draining me.

What should I do about this?