What should I do about dog frequently vomiting white foam and undigested food?

My 3 yr old 14 lb Shih-tzu/Bichon mix dog has begun frequently vomiting white/yellow foam. It’s not constant but at least a few times a day. She also vomits undigested food. Tried feeding less and Pepto Bismol, which DID seem to stop the vomiting for 24 hrs but she vomited again this morning, including the food she ate yesterday. We feed at 5 PM and our other dog isn’t exhibiting any of the same symptoms. She isn’t lethargic and definitely has an appetite. No diarrhea, no excessive urination, no bloating or apparent pain. She is behaving normally except the no-warning vomiting 1 to 3 times a day in the past week. She has been regularly immunized and on Nexgard and Heartgard. There is a stomach flu going around in humans in the area and I wonder if it’s possible it she’s caught a form of that although shows no other symptoms other than the obvious nausea and vomiting. Any ideas?