Have you changed food recently? Changing them over cold turkey, can do this to them as well. Trixie threw up a couple of times in the week that I switched her food from Royal Canine to a Vet Diet of Eukanuba. Even though I mixed the old and the new food for a week, she still had an upset tummy. I gave her Nutrical and water after she threw up. And she was fine!
But if it persists, please consider seeing your vet.
__________________  Trixie |