Lady had HGE three times in four months a couple of years ago.
They don't know what causes it, but toy breed dogs are prone to it.
Knock wood, Lady has been fine ever since I changed her to a homecooked diet and added probiotics.
Here are a couple of links for you:
Hemorrhagic Gastroenteritis (HGE) Canine Hemorrhagic Gastroenteritis (HGE) ~ Pawprints and Purrs, Inc.