Our old labby, Luke, had a reaction like that one year when we doubled up his rabies shot with his DA2PP shot. He also showed none of the classic allergic reaction symptoms, but panted, threw up, got diarrhea and wasn't himself for a while. The vet said it was a reaction to the vaccinations and since then we've always been careful not to get our dogs vaccinated with everything on the same day.
The good news is that he was right as rain again in a day or two.
How is Onnie now?