具體說明見內

If a dog vomits only a frothy, clear or yellowish fluid, it probably has a stomach problem, such as acute gastritis, but it also could have pancreatitis, peritonitis, or an intestinal obstruction.

And going through the motions of vomiting, but not bringing up any vomitus could be a sign of bloat (also called gastric dilatation and volvulus, or GDV), a very dangerous and often fatal condition in which the stomach twists inside a dog.

http://pets.webmd.com/dogs/guide/_dog-vomiting