Do you mean throwing up? Vomiting? That could be many causes unrelated to the treatment of the diabetes, but possibly too low or too high blood glucose if you are not monitoring.
Can you give the information the BJM asked about, the type of insulin, the dose, how long he's been on insulin any BG readings you have gotten either at home or at the vets. Are there any other medical issues going on? Did the vet do any other tests like checking for pancreatitis?