I am all too familiar with HGE. Lady had three bouts with HGE in four months.
Good for you for taking her to vet! You probably saved her life. The key with HGE is immediate supportive care. Hours can mean the difference between life and death.
Your vet was referring to her "packed volume" (red blood cell count). 55 is right on the edge. The first time Lady had HGE she was right on the line, too. She got subq fluids, an antibiotic and Metronidazole, too. She bounced back pretty quickly.
I've attached a few links for you. Bottomline is that they don't really know what causes HGE and Yorkies and other toy breeds seem to be prone to it.
Hemorrhagic Gastroenteritis (HGE) Canine Hemorrhagic Gastroenteritis (HGE) ~ Pawprints and Purrs, Inc.