
It could have been something you didn't see that has made her react & warn dogs off whilst on lead & it could be that she sees her role as protecting you against the other dog whilst she is attached to you by the lead, which is were the stooge or non reactive dogs come in.
Does she display this behaviour with other people have her on the lead ? that is someone other than your immediate family ? If she doesn't or you have never allowed anyone else to handle her this is were your trainer or other handlers could help away from the club & off your property. If it is only when with you or with your family that she displays this behaviiour you have a key to the reason she is behaving this way
I would certainly have a word at your club & see if they can set up some safe encounters for you. One thing you should try is not reacting at all & having someone else with you to distract her. I took back a dog that had gone to a show kennels & because he hadn't made it to show quality at 6 months had been left in a kennel with a dog aggressive male champion. So the puppy had learnt to be dog aggressive from the adult, when I got him back the first thing he did was attack my older dog(who was totally 100 % bombproof)by grabbing him by the neck my dog did not react & his mane coat protected him from harm. I obviously separated them. I then devised a training program wihich had my older dog on lead with a handler & me with the puppy on lead we approached each other & passed on the handler side on the sign of any aggression we both about turned & walked away, we then had one dog in the down & the other approach again on any reaction from the puppy & resulted in the approaching dog being about turned & walked away, no reaction & both dogs were treated & fussed. It took about a week of several training sessions a day to re educate the puppy & I of course was lucky that my older boy was a perfect stooge dog, they ended up the best of friends & after doing Schutzhund training with the younger boy he had an outlet for his drive & protection. He was always 110% with people & once trained he was non reactive to other dogs whilst being worked or trained or simply walked(part of the schutzhund training)
I have a feeling that she had no problem at the dog club because most of the dogs would not react to her because they were being worked & was probably not been receiving any eye contact or threatening body langauge from them, which is why I suggest that you ask the club for help away from the training club
It is difficult to advise over the net about behaviour problems as I cann't see the way your dog is reacting(eg bum up & head down & barking is not threatening in a GSD it is an invitation to play ! Whereas in a BC it can be aggressive but also might also be an invitation to play as well)
You could try using a head collar & ordinary collar to give you more control over her