sudden death

I breed rabbits. These past few weeks 5 does died suddenly. One day they seemed fine and healthy, they were feeding regularly. THe next day I find them lying on their side, with no strength to stand up. It's like their hind legs have been weakened. And they die the next day.  The common factor between the 5 does is that they all had about 10-13 bunnies approx 10 days old.  Could you pls help?  What could it be and how can I treat it? None of them were old. For all of them it was their first pregnancy.