Bloat or Torsion - the mother of all emergencies.
** Get them to a veterinarian immediately! **
There can be irreversable damage in less than an hour.
Alert your vet that you're on your way - Don't wait in-line - this is no time for polite manners -
push to the front of the line - explain your emergency - you need an X-ray NOW.
Your dog's life depends on you.
Symptoms can be subtle - one or more of these -.

Continual unproductive vomiting or retching (the dog may produce frothy foamy vomit in small quanties)
Salavating, panting.
Paces around continuously, or, lies down in odd places and just can't get comfortable
Acts agitated, whines.
Excessive drooling, usually accompanied by retching noises
Swelling or hardness in abdominal area (which may or may not be noticeable