sudden aggression

hi kimberley, thanks for you post.
Before we consider behavioral changes and training with your dog I would recommend taking her to the vet for a full check up. Such sudden changes in behavior can often be associated with pain such as illness, the beginnings of arthritis or a hard to notice injury, or with changes in sensory abilities such as hearing or vision.

I also have a further question about her aggression. When she is aggressive what action do you take and how does she respond to it?

It is good news that she is healthy and passed her vet check. her situation is unsuaul given that no changes have occured in her environment. However aggression with strangers and at the door is usually a sign that the dog is assuming a pack leader position and feels that it should be guarding both lower bonded pack members and the packs territory. For whatever reason your dog feels that she must currently take on this role, just because your dog is not being aggressive to you does not mean she does not feel like pack leader. You and your dog are closely bonded and she still considers you a close member of her pack. As a result reestablishing alpha-dog training along with desensitising her to strangers by giving time out for bad behaviour and praise and treats for calm behaviour should sort the problem.

Remember that in a wild dog pack the dominant pack member controls :
-access to food
-access to favoured sleeping areas
-any interactions with lower pack members
-access to favoured items such as toys

In order to show your dog her position as bottom of the pack you and anyone interacting with her can make use of these keys points, for example:

1) Your dog must be the last to eat at every meal and should never get treats from the table.

2) Your dog should never walk through doors before you. A good way to practice this is to walk around the house and make her sit at each doorway and wait.

3) If your dog is lying in the hallway or anywhere you have to get past make her move.

4) When you arrive home completely ignore the dog for 15 minutes. Don't look at her, talk to her or pat her. After this go to her and give her some quiet attention only as long as she is relaxed and calm.

Only interact with the dog on your terms. If your or someone else is petting the dog or playing with it and it becomes aggressive or badly behaved immediately remove your attention from the dog by either removing yourself from the area or moving the dog to another area. You should do this without displaying any emotion such as anger just be a calm but decisive pack leader and the dog will appreciate knowing where it stands. Similarly you can assert your dominance by not allowing access to beds and couches or only allowing access when you have invited her up. Also you can assert dominance by bringing out favourite toys when you want to play and removing them when you are finished.

Because your dog is responding to you when you take the lead and say "no" to her aggression I would predict a speedy return to good behavior after a short time back in training, all the best and let us know how you get on.