If you avoided obvious signs like audibly knocking knees and throwing coffee at the interview panel, it wasn't that. The odd 'erm' between sentences is fine.
There may be a few people who consistently interview well, but that's usually it: they interview well - and frequently - as they power upwards leaving a trail of partial achievement. Remember, interviewers can have bad days too: the school governor who asks a stupid question then looks across to the head to see if your muttered response is right. There's almost always one who could be an axe-murderer too. Probably is, in fact - you wouldn't have wanted that job anyway.
More to the point, the job was probably not yours - the application process turned up the candidate the governors think is best 'on paper'. If she performs as expected she gets it - you were there as back up. Remember:
* They expect you to be nervous.
* If they don't encourage you, then it isn't a nice workplace.
* You may well have been fine at interview; they just wanted someone else.
* Sooner or later the job will have your name on it.