If superfecundation (bred more than once over a period of days) occurred, it's possible that a late breeding is just now coming to fruition and that she may be in labour once again. The most expedient manner in which to determine if this is, indeed, the case is to have her vet palpate Whiskas's abdomen. Depending upon her overall health, "watchful waiting" might be recommended or, instead, a caesarian or spaying performed. Please respond with further questions or concerns if you wish.
That would be consistent with impending labour but it's also seen with mothers who have other things on their mind; in other words, mothers who aren't as good a mother as other mums.
Approximately 20% of neonatal cats die for a variety of reasons. The kitten's death isn't necessarily related to its mum's behavior. Yes, I think it would be smart to have her and the kittens examined if only for the usual post-partum exam vets recommend. This will give the vet the opportunity to carefully palpate your cat's abdomen for another kitten. Please continue our conversation if you wish.
Your other pregnant cat could certainly be a significant stressor to Whiskas and that cat should be kept away from Whiskas even to the point of confining Whiskas to an area that can't be accessed by other cats. A physical exam by her vet is likely to assuage your concerns. At this time we can only conjecture what's going on in your complicated cat conundrum.
It may have been a sudden change but it's not necessarily abnormal. Cats move their kittens around when a perceived danger exists.