I agree, but I also wanted to add that dog aggression is not uncommon, nor is it the end of the world in our breed. Boxers are descended from a type of dog that was traditionally used for dog fighting, so there's no surprise that some dog aggression exists in our breed. Additionally, it is very unnatural for wild dogs to socialize with strange dogs outside of their packs.
Given this, even though dog aggression can be expected from time to time in some boxers, it is very inconvenient to us as owners so we try to do our best to raise our dogs sans-aggression by socializing them, training them and establishing ourselves as leaders. These things go a long way toward preventing unwanted tussles when meeting strange dogs in public, but as responsible owners, we always need to be aware that things can happen during these meetings and be aware of body language while they're interacting.
I guess that's my long winded way of saying that it's normal for her to act this way (don't get worked up), and that you should speak with your trainer about recognizing posturing and dominant behaviors as well as practicing the NILF method in your home. This will, in turn, help her build confidence and take some of the stress of not knowing quite where she fits in in your pack structure out of the equation.
Good luck!