My new puppy sits in the same spot all day. Can someone help me?

I just got a new puppy, she is a 10 week old lab/greyhound mix. She hasn’t really been socialized, came to a rescue from a kill shelter (she only had like one more day) so I get her home, she won’t come out of her little dog bed we gave her. This is in the kitchen. In the other room we have a cage/house because my older dog is scared of thunder (it’s her “safehouse”) puppy goes in, but won’t come out. When she is inside she make a beeline for these 2 places.
Outside she is normal, she runs, she plays, wags her tail, but as soon as she get into the house it’s right back to these 2 places. She gets to the bed and becomes very lethargic. We have lured her out with treats, she’ll take the treat and quickly go back to her spot. She doesn’t really know what to do with toys either. She is also afraid of most humans, she only seems comfertable around me, I suppose that is because I am constantly interacting with her, but if I go to another room, she will cry until I come back, instead of getting up to follow me.
I know these things take time, and it’s only been a few days, I’m just concerned because I’ve had new puppies before and they were only scared for like an hour, and then the house was their domain.
The only upside to this situation is her comfort with being outside makes her not go to the bathroom in the house, she hasn’t had an accident yet. I just wish she wasn’t so scared. Does anyone have any advice I can use, I want my puppy to know she can run around the house and not be afraid.