Vomiting Shih Tzu

My 7 year old Shih Tzu was vomiting for 5 days.  He would eat and drink in the evening after I returned home from work & shortly after, vomit.  The food was undigested & contained clear liquid.  He acted normally--drank water, ate food, ran around playing and did everything he normally does except for the vomiting.  He had had no change in foods, but he is the type of dog that eats nails, paper clips, tacks, necklaces, plastic, and anything he isn't supposed to that he can get his paws on.  Since I have 2 children I never know what they might drop or leave lying around,  and; therefore, I am often pulling things out of my dog's mouth.  I took him to the vet and she did blood tests and x-rayed his stomach to see if he'd ingested something he shouldn't have.   Everything came back as normal.  She saw no foreign objects in his stomach, so she prescribed 8 tablets of Cerenia for nausea (1 a day) and a tube of some kind of paste to re-establish bacterial abnormalities in the stomach.  She also told me to give him Pepcid AC (1-2 tablets a day).  Then she told me that if he resumes vomiting after taking the Cerenia to call her back & my dog would probably have to have an ultrasound to check for ulcers or cancer.  To make a long story short, my dog resumed vomiting 4 days after he took the last Cerenia.  He still is taking the paste & the Pepcid AC.  I am waiting for the vet to return my most recent call.   Has anyone ever had this happen to their dog & found out that it was some chronic disease that can be treated with daily medication?  I am heartsick that something life-threatening could be wrong with my dog.