Answer unclear. Ask again later.
Kevin Feige has said that the post-credits scene (technically the only one, as the other one is a mid-credits scene) was footage shot for Civil War and that the scene will appear as part of the larger final cut of Civil War in some way (the final cut still has to be edited).
I won’t tell you exactly where it is, but that’s photography from Civil War. That is a part of the movie - it’s dailies from a part of the movie. Whether it will look exactly like that in the movie I think will shift a little bit, in terms of how we cut it, but that was footage shot for Civil War.
He reiterated this in another interview
this one actually jumps partway into “Civil War.” Marvel Studios head Kevin Feige confirms to Zap2it that the Falcon/Captain America scene actually takes place in the middle of that movie. “That is photography from that film that will play a part at a certain point in that film,” he explains vaguely.
In the comics, Civil War is kicked off by an explosion at an elementary school involving super-powered humans. Thus far, signs point to the event being a result of events like Sokovia, Wakanda, New York, and possibly another city being destroyed.
The Winter Soldier ended with Cap and Falcon on a personal mission to find Bucky. Where he has been found, and why is arm is stuck is left unexplained, and what exactly the "accords" are that are mentioned, is still unclear. A number of news sources (such as /film) are reporting that the "accords" refer to "The Sokovia Accords" put in place after the events of Age of Ultron, but I've yet to see any official statement from anyone involved with the movie.
Whatever happened "a week ago", is likely an event that takes place during Civil War.
The actual dialog in the scene is something very close to this (this is from memory but I paid close attention the second time I saw it -- @mike):
Falcon: This would have been much easier if it had happened a week ago.
Captain: We can't call Stark...
Falcon: ...because he wouldn't believe us.
Captain: And even if he did...
Falcon: the Accords might not let him help.
Captain: We're on our own here.
Falcon: Maybe not... I know a guy.