I agree that a few days into it is very early on, and probably premature to decide she needs to go back. She is trying to settle in, understandably you are anxious she does - and if she is picking up on your anxiety, it will be a vicious cycle (but one that can be worked on).
Forgive me if you are an experienced dog owner and know all of this - where a lot of dog problems start is that the dog is treated like a human, that the dog is equal and has the same 'rights'. But in reality, dogs need to have a clear pack leader in order to feel secure - and preferably that pack leader is YOU.
Can you see a dog behaviorist and / or take her to obedience classes? Most dog issues CAN be worked through - with the right training and right techniques. Often it can be as simple as US changing OUR (very subtle) body language. It sounds like she thinks SHE is the pack leader, and therefore her job is to protect her pack - i.e., you. She will probably be very over-protective of you if she believes she is the pack leader. It will also make her very nervous - as I said, dogs feel safest and calmest when they are NOT pack leader - it is a huge responsibility, and they are most happy if you are the pack leader, and all they need to do is tag along.
Dogs are happiest when WE are the clear pack leader. It can be as simple as 'allowing' her to go through the doorway before you, her sleeping on your bed, her not being made to move off your seat, and even feeding her before everyone else in the house has eaten. she will also pick up if you are overly anxious about her behavior. And that will make her more anxious.
Not being funny here - but is there ANY chance you might be pregnant? I have seen dogs behave like this if their owners are pregnant t- and that is even before the owner has realized she is pregnant - dogs can detect these things from very very very early in pregnancy (not meaning to freak you out, it is a long shot, but worth checking out).
A couple of links you might find helpful:
alba (about different types of aggression (yes, over protectiveness is aggression).
And a really good run down of how you know if YOU are the pack leader, and what you can do or stop doing, if your dog thinks it is the boss: (scroll down the page - very good outline of things like making sure YOU control the dogs food, where it sleeps, it's toys, etc):
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But do seriously consider a dog behaviorist.
Don't give up - it is very early days, and it can all be worked on! ;)
