I warn you now, my questions are probably going to seem odd...
What has her vaccination schedule been so far (age/vaxs given)?
What food are you feeding, mainly what is the protein %?
What was her life like at the breeders? What were the parents like? Was the breeder able to tell you anything about the parents' lines or even why she/he choose the parents for that litter?
What is her day like with you? Does she have free run of the house, etc?
I don't have Aussies, I raise Border Collies, but both very smart breeds and have alot of the same issues. For many years I did what the hubby called Border Collie intervention. I tried to help people keep their dogs in the home and when they couldn't or didn't want to try I took the dog in and worked with them and re-homed them.
My reason for asking some of the "strange" questions above: some breeds and individual dogs are more sensitive to vaccinations than others, rabies vax being one of the worst for reactions. I can't remember off hand for Aussies but for Border Collies they shouldn't have a food that is more than 20-22% protein unless they are working hard everyday in some fashion. Also some dogs (I had one sent to me from a humane society) have aggression issues while on commercial foods, it's more to do with additives and grains not a particular food. The way a puppy is raised while at the breeders can make a hugh difference too, especially if there are aggression issues in the lines to begin with. At 8 weeks when a breeder sends home a puppy, it's a big transition for them - especially for those who haven't been desensitized to that coming. It's so important to start right from the get go with showing the puppy that you are alpha. When I say alpha, show that yes you are incharge but not by pushing/rolling/hitting/etc. When I bring a pup in they spend alot of time tied to me this also helps with bonding. Most breeders don't give puppies free run of their house. There is a set area or kennel/run. So to teach the pups to live in the house and follow "House Rules" having them tied to me by a leash helps give me control over them while they learn. It helps to teach them that I am boss and I say when - when they go potty (helps us to keep an eye on their ques too), when they eat, when they play, when they sleep. Given to much freedom can be overwhelming and stressful which will bring out aggression behaviors. Your home is probably nothing like the breeders was and she's not feeling secure so she feels the need to be incharge (be alpha). This needs to be addressed asap as it will only get worse as she gets older and allowed to carry on this way. You will have to bring in a behaviourist or possibly your current trainer depending on their experience level - not only with aggression but with Aussies. Some trainers, even behaviourists I've met haven't a clue how to deal/work with the herding breeds. They misread the dog's ques because they don't fully understand the working dog's mind and mannerisms.
Some basic things to do right now: she works for everything. Nothing can be a free ride. She should have no bones or other highly prized items that she will fight to keep, till she is behaving the way you want her to be she doesn't get them. Everyone in the house needs to spend time with her tied to them. She needs to learn they are above her - not equal or below her. The one thing you can never do to Aussies and Border Collies is raise your hand to them in any fashion. They remember and it can cause them to loose confidence in you as a leader. If you have to do a correction, make it a quick sharp "No", (around here me raising my voice is all it takes, even for my two 8 month olds) and move onto something positive right away that you can praise/reward for. When it only needs to be a warning I use "hey or oye" and drop my voice an octive lower so they know it's a warning and then work on something I know they can do and praise/reward for it.
But if you could answer the questions above, it may give us all better insight into her.