For an entire litter to be stillborn, there's something drastically wrong. Was she in labor for long? My rescue girl Beta's contractions stopped and she needed to be rushed to a vet, and even then most of the babies didn't make it because they didn't come out in time.
For her to still be lethargic, I'm very worried that something is still inside of her, whether it's more stillborn babies, or a placenta, or who knows what... She NEEDS a vet right away. SQ had a girl with dead pups inside of her, and had she not have gotten her spayed right away, her girl would have went into shock and died.
Also, I've never heard of a mom stepping on a baby and killing it. Another rescue litter I had here, Trapper, she would literally trample the babies and they'd all just squeak away. I have pictures of her standing on top of them, and nothing happened to them even so much as a scratch.
I hope you see now why it's so important not to breed rats of unknown health/genetics. I'm sorry for the losses, and I hope your girl will be okay.