Is your house haunted? Do you need an old priest and a young priest? Just Kidding.
I am no expert, but...
Had anything changed in the house just before this? Anything like the A/C not being on until yesterday and she doesn't like the noise or something like that. How big is your house? Can you put her on a leash and walk her through all the rooms to see if she's more agitated in any particular room?
Almost anything can upset a dog, even something we (humans) can't see, smell, or hear. As for your older GSD, whatever is upsetting the rescue could be something that either doesn't bother the older GSD, or it's something he's gotten used to.
I kind of agree with Stosh. Get a crate but don't close the door. Just let her know the crate is a safe place for her. You might even drape a blanket over it to block out any sights and limit any sounds that might trigger the behavior. And, if you were able to lead her around the house and found she is less upset in one room, I would suggest putting the crate in that room.
I also kind of agree with Marnie. Put her on a leash and bring her in through the front door, but I wouldn't recommend locking the dog door. If she really cannot handle being inside and something really is scaring her, she really needs to be able to "escape" the situation. Trying to keep a frightened dog from being able to escape is not wise (IMHO). However, I agree not to let the behavior go on for too long.
No matter what, talk softly, remain calm, be very patient.
Isabella "Bella" Shedmeister RubMyBelly PrancyPants French - Black and Tan female GSD adopted 04/29/2012 (approx 4 or 5 years old)