I unfortunately know exactly how you feel, a few years back I lost two on the same day, they were both fine at 4pm, by 5 one was dead, and we got up the next morning to find another gone. The first mare was older and as she had gone straight down, no sign of a struggle, hay in her mouth, we thought she had suffered a massive heart attack, when the filly was found dead it was obvious something was far wrong.
It turned out to be poisoning from a sample bag of chicken layer pellets that had been in the barn, we still don't know how it got into the paddock, but less than a pound killed two horses.
The diagnoses came very quickly from the necropsy on the filly, that's when we were told that it was either Monensin or Narasin that killed her. At first they thought it was cattle feed, but at the time we were feeding all home grown feeds, so we knew it wasn't that, but then we searched and searched the paddock, before finding this little plastic bag, that had caused so much grief.
In the course of my research into this stuff I found that horses have died from eating feed from mills where they have switched from making cattle feed straight to horse feed, without cleaning down the plant between batches.
I don't know if the symptoms fit what you have there, in any case I hope you find answers soon