I want to say it could be a false pregnancy but I always urge that a dog going through what you dog is going through has an ultra sound to make sure there is not a dead pup in there. If she is pregnant and over due and due to her breed mixes may require a c section.
C sections are not cheap, spaying would have been a lot less expensive but the fact now is something is going on and she NEEDS to see a vet asap.
Courage is being scared to death and saddling up anyways. ~John Wayne