Are you on probation? If not, you're unlikely to get fired. And I just gave you an out for it being 3 weeks. Say you just happened to be thinking about the entire situation and it hit you that it was likely your mistake.

Hell, I'd be more likely to promote someone who was able to own up to their mistakes, regardless of the consequences, than I would be to fire them. Especially if they could've stayed in the clear and let someone else take the fall.