Have you tried just asking her politely how everything is going with her? I mean, I think you could politely say, during a car ride, "you seem a little quiet lately, is everything okay?"

I think until you ask her outright, you shouldn't speculate what could be going on. It could very well be something personal that is causing her to act this way. Something could be going on in her personal life that is causing her to sort of shut down socially.

Good luck,
Seesaw