Dog White Foam When Exercising. Your pet may spit up white foam after eating or exercise because his stomach has filled with air or fluid. Your pet may spit up white foam after eating or exercise because his stomach has filled with air or fluid.
Dogs love to run and play, and this can also cause dehydration. The stomach will become swollen and hard. Often your dog may even bring up white foamy mucous at the end of a coughing fit.