The dog I had before always threw
up after he ate grass. Like you said (margypops)
he would eat it when it was getting close to mealtime, when his stomach was empty. Then he'd throw up just the grass and a little mucus. Yet because he was very healthy all of his life, I thought it was just his way of 'cleaning out his stomach', to get rid of that little bit of mucus. I never thought it might be because he felt nauseous because he was hungry. But it could have been.