While it may not be "normal" behavior, it is most certainly not unusual behavior. Immature, ill, and stressed bitches will kill and even eat their puppies. This is more common in very young bitches and first-time moms. Sometimes this behavior is seen immediately after giving birth, sometimes after the pups start moving around (and, yes, teething). Mom should not be left alone with the puppies without constant and close supervision. She should see a vet ASAP to make sure that she is not ill: no infection, mastitis, malnourishment, dehydration, wounds on her nipples, etc. The safety of the pups and the well being of the bitch are equally important. If she is physically healthy, producing and adequate amount of milk, etc then she may be emotionally unstable enough to handle raising a litter. Make sure she and the puppies are in a calm, quiet, non stressful room where she can be watched closely. If you see her getting stressed (panting, "freezing" her stare, turning her head away from the puppies, growling, pacing, or trying to get away from the puppies) remove her from the puppies and give her a "rest period". Do not reprimand her (this will increase her anxiety and tension), just let her relax away from the pups for a while, then see how she is when she is returned to them. Some bitches react negatively to the litter when they begin to feel "trapped" and can't seem to get away from them, but do better when given regular breaks away from them. If the puppies are in danger, of course, you may need to separate Mom and pups permanently and do the raising by hand from this point on. Hope both Mom and the little guys are okay!