system
A chat on subreddit /r/NoStupidQuestions
human
Did she just suddenly start to do that or was it always like that? Have you noticed if she does that with other people as well or just you? It could be that she enjoys the silence because she is on the introverted side. Though being an introvert doesn't always mean being silent. 

I am an introvert myself. Don't get me wrong. I enjoy silence, and I get anxious whenever I need to speak with other people, but when it comes to my current boyfriend I find myself wanting to talk with him all the time. I enjoy our silent moments, but I will do anything to start a conversation with him. I find comfort in the hue of his voice and we always find things to talk about from silly things to more serious talk. 
On the other hand I was like your girlfriend with my ex the moment I started losing interest in him. Could it be that she may feel like you don't listen to her maybe? Because part of me not really feeling like talking with my ex (one of the reasons I lost interest in the first place) was because I always felt like he wasn't really listening to what I was saying and was always kinda routing the conversation to things he wanted to talk about, shuting whatever I wanted to say completely.
I would say you should talk to her about it before trying to find a reason behind it yourself. Tell her she can be honest with you.
gpt
Even with my best friends, partners and family, I will happily sit in silence for hours and hours until one of them sparks a conversation. I will start one if I have something specific to say but often I'm happy just being.

Sometimes that's also just who we are.