How horrible. Was there blood, signs of a fight? Could she have choked? I had that happen to a litter of pound puppies, or at least that was the conclusion the vet & I came to. I found 1 of 5 dead, a little vomit, I think it got a bite of kibble & one of the sibblings wanted to play & jumped on it causing it to choke. No signs of anything except one little bit of vomit from choking
I hope you find out forsure. Poor BooBoo.