My healthy 10 yr Shih-Tzu is vomiting once every day for a week .It is just yellow foamy bile coming up.....?

He eats great, bowels are perfect and exhibits no other symptoms. I have a date w/Vet tomorrow but am wondering if anyone has had this problem. Could this be anxiety? He has no reason for bad nerves but I can't figure out what is making him vomit when he acts otherwise very healthy. Thanks.