Congratulations, you're seeing firsthand the benefits of fresh food! What's happening is that your dog is quickly metabolizing the fresh ingredients, and getting a burst of more readily available energy.
Just like you'd likely feel lethargic after eating a fast food meal, your dog may be used to food with ingredients that don't provide them with the optimal digestibility.
That all changes when your dog begins eating right!
Keep up the good work as a pet parent, and feel free to share their story with us!