your Vet is probably right a bug , but not all bugs respond to anti b's
Just to tell you of a pig that l had with a diminished appetite, (lucky to get him to eat 1-2g of dried food a day , and would not touch veg at all !
he went on the Vet meds merry-go-round , and syringe feeds but nothing worked and the situation lasted for a long time , and he lost over 30% of his body mass
when l first took him to Vet l asked for a poo analysis, but vet just did a float and microscopic ! and said all was well , and proscribed Batril for pos tummy bug !
wasn't till l 5 weeks later that l requested a full spec poo analysis by the lab , did it revel that his tummy lining was full of Giardia a microscopic protozoan parasites (one celled organisms)
apparently several different fecal samples may be needed for a positive identification. Special stains may be needed, as the organism can sometimes be elusive.
and althoug one of the most common symptoms are runny poo's , it is not always the case , and in the case involving my pig , the only outward symptoms were no appetite (possibly due to gut irritation )
after the correct meds and a lot of time , he recovered , he is now eating properly and regained all his weight ,
not saying this is what is wrong with your pig ! just something to consider