I've fostered nearly 100 dogs. Sometimes they take some time to adjust and get comfortable. Some dogs even take a couple of days before they start eating. As long as she isn't getting dehydrated she'll be ok.
Take her to the vet if she is still acting lethargic and refusing food in a few days. If she doesn't seem to be in pain or running a fever, just give her some time to adjust. She is coming from a highly stressful environment. Changing homes is stressful in itself. Time and patience is likely what she needs.
Relax and enjoy your new addition.

Thanks for rescuing a dog in need!
PS Common possible mistake - Be sure not to stand over her food bowl. It will make her less likely to eat.