Hmm. Well, I guess I'm just odd. :D But on thinking about it some more, I think the most plausible theory is that Kahvi, looking at the sword, realized how much it was shaped like New Moon, and realized that that's why she had made the mistake-- cutting Nightfall free instead of Cutter. Their swords would feel the same under wrapstuff.
Which, it seems to me, is a subtle reference to how much Nightfall and Cutter are alike, personality-wise. Kahvi, thinking she was getting Cutter, was in a way getting the female version of him. And it was more than she could handle!
But no matter how often I look at it, I can't see Kahvi as just suddenly feeling nauseous, so coincidentally, at the moment she's staring down the blade of that sword. It may be just me, but there it is. :)