I am so sorry for your loss, I would talk to a repro vet and see what can be going on, I also don't breed back to back, I do one skip one skip, it gives my girls time to recup and ME too..
best of luck and keep us informed, I belong to the puppy repro group in yahoo too, is a very Good group with LOTS of opinions and experience.
hugs,
CArmen in nj