
Don't worry, this is probbably just one of those things as others have said. Your dog could've been having an off day, the other dog may have been sending out signals, the vets is full of all different smells and emotions, it could have been any number/combination of things.
As somebody else said, with an out-of character display like that it will be worth keeping an eye on her for the next few days to make sure she isn't in pain/comming down with something.
Honestly, don't let this get to you, it doesn't mean it will happen again. Stay confident with her, if it happens again you can deal with it as an when, just don't make life hell for yourself over-worrying.
My pup is always getting growled/lunged/snapped at at the vets (I think other dogs are intimdated by his size). Both of these dogs have never done that before, in fact 1 of them was a 9 year old champion Boxer, so not an immature unsocialised dog (the owners were very shocked), but it just goes to show that even the best behaved dog can have a bad day or meet a dog it relly doesn't like.