The only way is the vet doing a neocropsy to determine the reason otherwise it's a guessing game.Kind of sound like a bad case of hypoglycemia.

Sounds like poison something they got a hold of,or Parvo has similar symptoms.

Everything that the folks said about poisoning is valid. You would need a necropsy to determine exactly what happened... the other thing that I could think of is heat stroke... small dogs are easily overcome by heat... even if they are outside in the sun for an hour... it can cause vomiting, vertigo, etc...