So very sorry. My Mom just went thru the same experience. You will probably never know exactly what happened-these symptoms can be soooooo many things. Just eliminate everything thats humanly possible to protect other doggies or cats or kids (anti-freeze, spiders in bad places, rodent poison etc.) Time will heal.
So sorry Andy, we just watched "Marley and Me" the other night, and got so upset thinking about losing one of our dogs. We told them all that they weren't allowed to leave us, but I'm not sure they understood.
I strongly recommend what gatorgal said, make sure there is nothing around that can do any harm in the future.