I think I read this story some time in the 90s or early 2000s. There was some type of disaster in America. The country was decimated. Almost no one was left alive. I don't recall if everyone died or if they were forced to evacuate over a period of time. But I believe most of the country was depopulated. The reason I call it a quantum calamity is that the walls between quantum realities had dissolved with increasing severity going deeper into the country.
There are two scenes that stuck out to me. The first is there were two main characters: a man and woman. They're creeping through a field and they come upon a couple having sex. I seem to recall them remarking on how stupid it was for them to be distracted having sex in a hostile territory. They creep up behind them, attack, and kill them. Only then do they notice that the people they just killed are themselves.
The next scene I remember is they make it deep into the phenomenon and encounter a man in an office. I think he's dressed in a suit. I think they remark or think how supremely uncomfortable and/or unnatural he looks because he keeps moving in strange and awkward ways apparently trying to get comfortable. He either notices them noticing his discomfort or he brings it up himself. It is revealed he's not actually human. He's some entity, probably from another quantum reality or even between quantum realities (?) who assumed human form to attempt to contact humans and end the disaster (?). It's also possible he just got trapped here. I don't remember.
I have to paraphrase what he says, because I only recall the gist of it. He said something to the effect of "As uncomfortable as I appear to be to you in this body, I can't begin to understand (?) how uncomfortable you must be in your mind." I think he was referring to them dealing with the shifting realities, which I believe was spreading, which was why they were journeying to America in the first place. I could be wrong though.
That's all I remember.