Three kittens dead, one vomited a massive amount of blood, vets baffled.

TW: this is a bit graphic and the kitten who vomited this passed on after being at the vet for the night. I’ve been fostering 6 week old kittens who at first glance only had eye infections, and were okay otherwise, besides two of them looking skinny. One had to be PTS after going from okay to very bad and then going into a coma. Two days later his brother who was doing okay and eating started to become limp, went to the vet and the suggestion was to PTS after he seemed to be in a coma too. Just as I come home after the second one being PTS I feed the remaining two kittens, they gobble up wet food and minutes later one vomits a large amount of blood. Rushed her to the vet, vet kept her overnight and she sadly passed this morning. I have one remaining kitten who seems healthy for the most part but I fear for him. Vets are baffled and rescuers too. What can cause a seemingly healthy kitten to suddenly vomit blood like this and deteriorate? Only weird signs in this one was a very bloated tummy but she was eating and drinking. All been dewormed and on Flagyl.