this is a difficult situation because there are a couple of factors to take into consideration. Dogs around 18 months or so start becoming more dominant
and seek to establish their place in a group of dogs when around them. Usually a well socialized dog will greet another dog, go on about their business or start playing or becoming better acquainted.
However, once a dog has a bad experience with more aggressive dogs, they often will become scared of other dogs. In some cases, the bad experience can be the result of a dog continuing to act like a puppy despite now being an adult. This will cause other dogs to reprimand them with growls or nips. In other cases, the young adult dog might have body language that indicates dominance, aggression or even challenging behavior
which can trigger the aggression by another dog. In others, it has nothing really to do with your dog and is just a case of aggressive or unsocialized dogs attacking yours.
I'm going to give you some sites that discuss body language so you can better determine the communication going on between the dogs.
It is likely your dog is now a little scared and in turn is becoming aggressive toward other dogs in an attempt to keep them from becoming aggressive first. So that likely contributed to her reaction to her playmate. However, her behavior may also be relatively normal dominance behavior that has been affected by her bad experience.
To correct the behavior, you need to keep her on a leash and monitor her body language and behavior. If she displays unwanted body language (aggressive or overly dominant right off the bat, reprimand her with a short tug to break her concentration and then a firm low toned NO to indicate it is unacceptable. When her body language is relaxed and friendly reward with nice calm praise and even calling her to you and rewarding with a tasty hot dog sliver. Try not to tense up as that will send the wrong signal to the dog and make her tense.
The BAT method may help as well. Read about that here:
Continue your training practicing daily even though she knows the commands. Training helps keep you the boss and your dog will then see that you are the boss. As the boss, though, it is your job to protect her and now you have to gain that trust back by being sure to not allow strange dogs close to her for a little while and socialize her extensively while providing positive reinforcement for desired behavior.
Try finding well socialized non aggressive dogs to socialize her with and then gradually reintroduce her to other dogs slowly so the bad experience is not repeated.
I hope this information is helpful to you. If you need more information or clarification, please reply and I'll get back to you as soon as possible. If you are satisfied, please take the opportunity to rate.