
First of all, take her to the vet, just to confirm that there is no underlying illness, but this sounds like a full-blown phantom. The best thing you can do to help her get over it quickly is to take everything soft out of her 'den', take away her cuddly toy, and prevent her from this 'nesting'. It sounds harsh, but the longer you let it continue the longer it will, and the more upset her hormones will become. Don't let her sit and brood, but take her out and about with you, keep her active, lots of walks, 'jolly' her along. She is likely to produce milk, so watch out for mastitis, because if this starts you may have a very unwell bitch.
If it is indeed a phantom, please don't indulge her in it - she will be much happier the sooner she gets back to normal.