Just wanted to add:

Our original flight BNA>DCA was only delayed ~15 min; no reason was given for the delay (the flights were US 3334/2050 in case anyone is interested). This, apparently, made our connection illegal and we were told that we would "likely be stuck in DCA" as they could not get us to BOS from there that night if we missed the connection. Essentially, we were told that it was either a night in DCA or a night in BNA.