If it's her first litter, it could very well be cannibalism. It would account for the missing pink, and she may not have begun to eat the dead ones yet... I had one female who, with her first litter, would chew the head off one.. Then chew the skullcap off another... Then move onto another.. Eat one until there was nothing left but some skin... She rarely sat down to eat the entire baby in one sitting, though she had no problem finishing off another girl's litter a week before this, without leaving any evidence... (That female, of course, has since been culled--I let her have a second litter, and she ended up cannibalizing that one, too, making a total of 3 litters dead at her paws.)
If this is the first litter your colony has had, leave the girl alone and just wait to see if she eats her next litter. She could be a down-right cannibal, or she could just be stressed by her first litter.
Of course, that's just my take on it. This could very well be something completely different.. This is just my experience on the matter. =P