Hmm, something doesn't seem right with her physically. I agree with the others. Take her to the vet and check to see if everything is physically all right with her. Because she was found starving and neglected, she could very well have some health problems you haven't found out yet.

If it is not a health problem, it sounds like your pup is being territorial. Rat terriers are, quite frankly, terriers. For that, they are feisty little creatures and require a bit of firm, loving authority. Have you heard of Nothing in Life is Free? It's basically a method that you use on a day-to-day basis. The underlying message is that everything the dog wants, the dog must earn. If the dog wants to eat dinner, he must sit. The same goes for toys, treats, etc. I would also agree with enrolling in obedience classes. These two combined will often nip most, if not all, your problems in the bud.

Good Luck!