Thanks for the additional information. It is helpful.
It sounds like your girl is a bit dominant. If it was a recent occurrence then I might think it was medical in nature but since this is an ongoing issue, it probably isn't medical in nature. If it was, I'd suspect a thyroid issue since they cause behavioral changes and sudden aggression.
Dogs are aggressive for a number of reasons. It might be that they are fearful of people or other dogs and thus are aggressive before the person or other dog can be. In other cases, a dog is aggressive in order to dominate the people. Other causes could be that the dog feels they are the alpha member of the pack and as the alpha member they must protect the pack (you) from threats.
In addition, owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that he is justified in his aggressive stance because you are obviously worried about the person or dogs. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the person or other dog.
You need to establish total control over the dog. You must be the boss. Actually all the people in the house must be the boss including the little ones.
You will need to have her obedience trained. If you can, I would do group classes and let the trainer know of the problem your dog has.. Before you can get into classes, I strongly suggest you start obedience training now. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
This obedience work establishes you as the boss and as the boss it is your job to protect the dog and not the dogs job to protect you. Now the dog will still protect you if something should happen because you will be protecting yourself and as part of the pack, she will follow your lead and protect as well. But if you are calm around someone, she will see that you are ok and there is no need to help. This is why being the boss works well.
Tiny hot dog slivers work best as treats. Each time a dog oveys a command (even if for treats) they become a little more submissive toward the person giving the command. So all members need to participate in training so she won't growl at them either.
It will be helpful if you can find someone with a dog to help you once you have your dog listening to commands consistently. What you will do is have your dog on the leash. You will have your helper off in the distance. Your helper will gradually move their dog a bit closer to you preferably walking past your position in the distance. As long as your dog ignores them, you can give your dog praise and a treat. The second you see her fixate on the other dog or show any other sign of aggression (hair standing up, etc.) give your dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take your dog long to realize you will not tolerate the aggression and that if she ignores the other dog, she gets treats. Once this happens you can repeat the training moving the other dog closer until she is no longer trying to lunge or growl at other dogs. You will need to practice this when you and your dog are walking as well. Once she is doing well with dogs, start with a human helper that she tends to growl at all the time. With each person you use as a helper, the training will go a bit quicker.
You will also want to keep a leash on her at all times initially to grab if she should disobey. I think you will be pleasantly surprised at how well your dog does with training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.
Unfortunately, at this point, I would seriously consider having a behaviorist anyway available to evaluate the dog and determine the aggression level. In the meantime, institute the techniques I have recommended.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.