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Getting another dog may have actually made the situation worse. Often when you add another dog, a current dog will feel like they have to prove to the new dog that they are the boss and thus they end up showing the new dog that they can boss you. You should never use a shock collar when a dog is showing aggression. They just associate the shock with you and view it as an attack. They can be good for teaching a long recall so it won't be a waste of money.
What has happened is that she has become an adult and likely didn't have a lot of training as she was maturing. As a result she decided to challenge you to become the boss of the family. This frequently happens when there hasn't been a lot of training when they are young. They believe that they can control you with growls and nips and often it works because humans tend to back off a growling or nipping dog. So the dog can control you and that makes them the boss. Once they feel they are the boss, then when you do something they don't like or attempt to make them do something they do not want to do, they will reprimand you. Their reprimand is a growl and if you ignore that a nip. Often they don't even have to nip as their snarling and lunge is enough to get you to stop. I see this usually around 12-18 months of age though some dogs do start this a little later.
Before I go forward, I do need to warn you that some medical issues can cause sudden aggression. You can read about these below.
You can correct the situation but it is going to take work if it is not medical in nature. I know you took her for some obedience classes but what she needs is extensive positive based obedience training. It doesn't have to be a formal training class but it does have to be formal structured training every day. In other words, pick a time that you can train every day preferably more than once. Then you will pick a command and practice it over and over every day until she obeys that command the first time every time. At that point you can add another command and work on that one until she is completely trained.
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.
Now obedience training works because only the boss is allowed to reprimand other members of the family group or pack. Your dog will become a little more submissive to the person giving the commands each time she obeys you even if she obeys just for the treat. Don't use boring treats. Use tiny hot dog slivers. You want her to want to obey you so she slips into that submissive state. Over time with the daily training sessions, she becomes submissive to you again and as I mentioned, submissive members don't reprimand the boss. All family members have to do the training. You can have one person start but as she learns commands, have others work with her as well.
Keep a leash on her initially and use it to give a short tug and firm NO if she growl or behaves badly. In addition, use it to remove her from furniture if you allow her on it.
In addition, if the situation is not improving using the techniques I've suggested, 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.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.