I'd call my vet, and give him the exact kind and weight of the chocolate. The chocolate doesn't always cause stomach problems it can cause nerve problems, that won't be seen for three days. It's much worse if it was dark chocolate. Using a dose of 100 mg/kg as the toxic dose it comes out roughly as:
1 ounce per 1 pound of body weight for Milk chocolate
1 ounce per 3 pounds of body weight for Semisweet chocolate
1 ounce per 9 pounds of body weight for Baker's chocolate.
Dogs and Chocolate Toxicity