I am not sure what happened to you, but it sounds like you had a wonderfully empowering and exhilarating experience.
Perhaps you reacted the way that you did to the unplanned situation because you felt that you had no other choice but to step up to the challenge on behalf of your colleague--whether that was out of loyalty, friendship, professionality, career ambition, or because of your personal moral code?
If it's true that you felt that you had no choice but to do the best job that you could possibly do, on that day--even though you did have a choice, in all honesty--then it could be extremely valuable to isolate the specific reason, or reasons, for why you felt that way. Of course, I realise that you didn't know what the reason was at the time of asking us for help, but I thought it potentially useful to offer you a few possibilities anyway.
I don't know whether the things that I've written will be helpful, but I just wanted to try my best to answer your question in some fashion. As I said, I do not know what happened to you, but I wonder whether you even need to have a precise explanation, in order to draw on the experience as a source of courage in the future.
Whatever the reason for your sudden act of bravery, you must surely have proved to yourself that you are braver than you think, under certain circumstances. And you know how it is with brave people; they tend to say "Anybody would have done what I did, given the same circumstances."--but we also know from our experience of the world around us that that isn't quite true! So perhaps it is OK for you to internalise the idea that you can be a brave person, sometimes, despite your anxiety?