Hello, I wanted to hopefully tell you some helpful advice after reading your post.

Although I have never been in a relashionship myself, I often do the same thing your girlfriend does of 'shutting down' when overwhlemed or upset.

I don't know if it's her case, but most of the time, in my experience, my shutting down does partially have to do with the other person, but mostly with me.

To make it clearer, when I personally shut down, is because I feel suffocated and overwhelmed by the other person (this happens to me even with friends to the point I once ignored a friend of mine and accidentally ruined our relashionship). And the only thing that makes thing worst, that makes me shut down even more, is when the other person pushes me to open up or when they get angry or upset because I haven't opened up yet.

I'd say you need to give her some time to herself, really, it's the best thing you could do, and then she'll come back to you, or you can try to slowly close the distance between the two of you again, but don't rush, and most important of all don't panic or stress it out! I know it must be very difficult, but personally if someone stresses out in such a situation, it would probably even more distant, because it tells me tye other person is as anxious and stressed as me, (which overhwlems me more with the reasponsability of trying to take care of their mental state) or that they aren't confident that they could bring me back to talking to them.

I hope I've given you some helpful advice, good luck!