[She knows enough about him. Knows so well how he always puts everyone else - her, although it's not like that and it never was - before himself. Remembers, for lack of a better word when those memories are false, all the times she's called him out for downplaying or hiding an injury, or sadness... Even if his words just now hadn't been completely unambiguous, there's no way he can make her believe they were nothing. This is too familiar.
As Bucky keeps talking, her eyes slowly grow wider as it completely sinks in. She hears perfectly well what he's saying but right now she doesn't care, and she cuts in almost before he's even done speaking:]
When...? Falling out of that train? [She remembers. And, oh, he was doing the same thing back then, trying to convince her it was nothing important, and she complied because she was so anxious to re-establish distance...]