Take her back to the shelter/rescue. It's only been 3 days and there have been multiple serious negative interactions. It is far better for everyone, the shepherd included, to not be in this situation.
If you decide to adopt another dog, ALWAYS have your current dog meet them on neutral territory - shelters often have special "meet and greet runs". If it is "not going well", do not adopt the dog. A general nervousness in a shy dog is OK - as long as the other dog respects that. But any more than that, and it is not a good fit.
If you decide to adopt another dog, ALWAYS have your current dog meet them on neutral territory - shelters often have special "meet and greet runs". If it is "not going well", do not adopt the dog. A general nervousness in a shy dog is OK - as long as the other dog respects that. But any more than that, and it is not a good fit.