Without knowing her in particular, anything I say will be conjecture.
But at least with me, sometimes I just get into little 'funks' - little moods - where I withdraw and I don't want to be around anyone, and it usually doesn't have anything to do with a specific person. While I'm in that mood, I'm aware that I'm in the mood, so I tend to stay quiet because of that. It's usually tied to me being self-reflective and going into analysis mode. I tend to distance myself from relationships in general at that point, just til I get more solid again. And..what I say when I'm feeling that way, is more likely a reflection of my present mood, which is why I don't say anything. I don't want to say anything that I'd regret, that comes out due to sheer irritability, or whatever.
So it could just simply be one of her little bouts of introversion, and nothing more. I wouldn't worry too much about it, or push her too much to divulge. She'll divulge if she needs to. But maybe try to keep her from introverting too much...keep her active in the world, and try to do activities together.
Sometimes when I'm aware I'm closing myself off, I wish deeply that I had an extrovert to pull me out, to keep me from going too far into my brain. So...activities!! Be present and available, but don't dig too deeply. She very likely will sort through whatever she's thinking about, on her own, and in her own time.
And, because she probably DOES carry wounds from past relationships (don't we all?), I'm sure she will tippy-toe carefully into it, or go, then retreat a bit, assess things, then move forward again...just being more cautious.
But again...simply conjecture based on myself. I think though that INFJ's are pretty complex anyway...so I have a feeling there are a lot of individual nuances.