Maybe she's just exhausted from carrying around a large litter of pups. Or, just like humans, she may be in labor soon. I had my daughter almost 3 weeks before the due date. Doctors/Vets can NOT pinpoint the exact due date. Maybe she's ready and is trying to let you know or as I said, just exhausted. Comfort her, make a 'nest' for her with old blankets, towels, etc...and show it to her as much as you can. She'll feel very comforted knowing you care and knowing you won't be 'mad' if she does this...especially if it her first time. She needs you.