Hi, Maybe the food, maybe something else. When you switched foods were the ingredient list very different? People often change foods not realizing they are not getting anything different.
Are you using heart worm protection? Just tossing this out there for thought.
FYI
Ivermectin has been proven to cause the following side effects: Vomiting, diarrhea, seizures, paralysis, jaundice and other liver problems, coughing, nose bleeds, high fevers, weakness, dizziness, nerve damage, bleeding disorders, loss of appetite, breathing difficulty, pneumonia, depression, lethargy, sudden aggressive behavior, skin eruptions, tremors, and sudden death. Though a minority of dogs experiences these reactions, they are seen in many breeds. Veterinarians also report that many dogs also get stomach and intestinal upsets, irritability, stiffness and just feel rotten for the first 2 weeks after each monthly dose of heartworm protection. An American Veterinary Association report on adverse drug reactions showed that 65% of all drug reactions reported and 48% percent of all reported deaths caused by drug reactions were from heartworm preventative medicine.

Keep in mind the stats at the bottom are from "reported" incidences and I believe that the reactions and deaths due to this drug are probably much higher. The reaction doesn’t have to be immediate either. A dog can be seemingly doing fine on heartgaurd or any other heartworm prevention containing Ivermectin (Ivomec) for months or years then all the sudden fall ill and neither the vet nor the owner would suspect the medicine