Irene,
It sounds like she absorbed the litter..
I had this happen to one of my females last Thursday! She was due the 21st of this month.. Last Wednesday night when she went to bed and her belly was huge and then Thursday morning her belly was flat.. My vet done an ultrasound, but they were all gone.. My vet also gave her a antibotic injection and sent her home with Clavamox, just incase of infection! Please call your vet!
I will not breed this female again last winter she absorbed part of her first litter and the other two puppies passed away..
Melanie