Littlelady,
According to a presentation entitled
Avoiding Vaccine Reactions in Dogs and Cats by Dr. Craig E. Greene at the 2003 World Small Animal Veterinary Association,
Avoiding Vaccine Reactions - WSAVA 2003 Congress,
"Immunologic reactions that can develop following vaccination can be categorized as to the four classical types of hypersensitivity reactions. Type I reactions involve an interaction with cytophilic IgE and antigen with resultant degranulation of circulating basophils and tissue mast cells. In the dog, signs are facial edema, hypotensive shock, weakness, dyspnea and diarrhea."
It is certainly a possibility that he experienced an adverse reaction to vaccination. I hope he's better.