Do get one of the dogs autopsied.

A friend of mine, Bonnie, who lives in very rural South-Western New Hampshire, had one of her dogs sicken and die, quite suddenly. She was bereft, but worried about her other two (all rescues, great dogs), and because until that moment Tucker had been a healthy big galoot, she really needed to know. She had driven down to the Grafton, MA Tufts Veterinary Hospital to try to get a diagnosis when he was suddenly so sick, and they agreed to do an autopsy, both so she would know, and so they would know, themselves.

Tucker had been poisoned. Some farmer had probably put out poison for coyotes, is all they could figure, we all just hoped it wasn't intentional. Only Tucker was affected because the other two were very picky about what they ate, so he got it all for himself, poor guy.