Thanks for the additional information. It is helpful. Now one possible cause for the aggression is a medical issue. Things like hypothyroidism which labs are prone to as well as springers is often a cause of sudden aggression in dogs. You can read about this and other medical causes on the following sites.
If there is no medical cause for the aggression, then it is strictly behavioral. Dogs are aggressive toward other dogs for a variety of reasons. It might be that they are fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is aggressive in order to dominate the other dogs and be the alpha member of the pack. 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 (other dogs). In this case, I suspect that it is a combination of the last two since she doesn't seem fearful.
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 she is justified in her aggressive stance because you are obviously worried about the dog. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog. I know you are trying hard NOT to do this but I know how hard it is. Adding a basket style muzzle can help with your possible tenseness or worry and also prevent her from actually biting. It may even help her realize that they are not trying to mount her (dominant gesture) and don't plan on doing anything else. The problem with allowing them to sniff her is that if she doesn't agree to it, she then feels you are not protecting her since you are "allowing" them to disrespect her.
If she is not spayed, have that done. If you let her on the furniture, you need to stop that. Dogs that are allowed on furniture tend to feel that since they are elevated to your level or higher if on your lap, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack. If you need hints on how to accomplish this, just let me know.
Continue your training classes and be sure to practice 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.
Obedience training serves various purposes. It helps a dog learn what humans expect of them when they state a command which leads to self confidence and less fear. Each time a dog obeys a command, even if it is for a treat, it makes them a little more submissive to that human in the future which helps with dominance aggression. And since it is the leader or boss who is responsible for protecting the pack, if the dog is made submissive with training, you are responsible for protecting him, so that can reduce aggression due to fear and dominance. The obedience and NILF programs are designed to establish you as the boss.
It will be helpful if you can find someone with a dog to help you work on this issue. 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. Be sure it is a dog that doesn't typically ignore her. You want her to display the unwanted behavior so it can be corrected. 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 at other dogs. You will need to practice this when you and your dog are walking as well. Do not allow the other dogs to sniff her. You do not have to totally move her away from them, but don't allow someone to let their dog sniff her rear at all initially. The idea is to get her to ignore pushy dogs that are lunging toward her and reward her for that behavior. You will work on the next step after she is more comfortable ignoring others.
Once she ignores pushy dogs, then you can work on nose sniffing. Keep a muzzle on her for this part. Only allow a quick nose sniff and then move on, rewarding desired behavior with hot dog slivers or liver slivers. Give the tug and NO if she tenses or otherwise reacts. Remember you are the boss and you are not going to allow other dogs to sniff her rear. If necessary, you want to move her away the minute anyone seems to think they want to sniff the other. While it is somewhat usual for dogs to want to sniff each others rear, a very dominant dog won't take kindly to another dog initiating that behavior, so it is best not to allow it until the dogs are more comfortable around each other. If you have the muzzle, you can even try some interaction off leash and see how she reacts. That might give you a clue as it if you are contributing to the situation inadvertently.
Now I'm also going to give you a site that discusses leash reactivity and one that discusses bat training. It may be helpful in this situation. If hormones are playing a part, spaying will help since she will not need to dominate to be in a position to reproduce since she won't be able to.
You might want to carry a hooked umbrella with you. It can be opened and put between two dogs trying to get to one another. The hooked end can be used to grab a collar or leash end and keep out of the immediate area of teeth. A large number of bites occur when trying to stop a fight. At this point, controlling her reaction will be a plus. I also want you aware of body language of all dogs involved so I want to give you some sites that discuss dog body language. You may find she is actually reacting to the other dog. If you notice her tail up in a dominant posture, gently brush it back to a neutral position. It sounds weird, but this simple trick often puts them back into a less aggressive mood.
In addition, if the situation is not improving using the techniques I describe, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
I hope this information is helpful to you and you are satisfied with my response. 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 .