It sounds like a contact allergy to me. The dog's stomach is touching something it is allergic too.
Like Ruth said, grass or weeds that may (or may not) have chemicals on them.
The fabric of your couch or type of carpet you have
Cleaning products used to clean carpets or floors
Fabric softeners, dryer sheets, or laundry detergents used on the dog's bedding or other items the dog is frequently in contact with.
Topical flea products used on the dog
And the list goes on
I do not trust a vet who wants to hand out drugs and medications without attempting to find out the cause of the medical problem. I would recommend going to a holistic vet that will help you get to the root of the problem.
