I;; HAVE A CHIHUAHUA AND A JACKRUSSELLE WELL THEY GOT TOGETHER NOT TOO LONG AGO AND THEY HAD 10 AMAZINGLY BEAUTIFUL PUPS. WELL THEY WERE FINE UNTILL THEY GOT ABOUT A MONTH AND ONE PUPPY WAS DOIN FINE AND THEN THE NEXT DAY IT GOT REALLY ILL, AND STIFF. I TRIED AND TRIED TO HELP IT BUT IT DIED A HOUR LATER. THEN MAYBE 2 WEEKS LATER ANOTHER ONE GOT LIKE THAT WHAT COULD I DO TO MAKE SURE THE OTHER ONES DONT GET BAD. ITS HEART BRAKING TO SEE THEM SUFFER!!!
When you say they got really stiff - were they stretching their legs out & putting back their head? If so, they may have been going into hypoglycemic shock - 10 is a very large litter for either Chihuahuas or Jack Russells - mom may not be able to feed the whole bunch. I suggest that you take mom & the pups to your vet for a check up - and talk with the vet about the possible need to suppliment feed the pups.