Get it into cold water or give it a cold-water rubdown immediately to lowers body temperature. Then take is to your veterinarian for a through examination. You probably meant well by taking your pet with you, but do your pet a favor. Next time, leave it at home. Even small amounts of theobromine, a naturally occurring alkaloid found in the cocoa bean, can cause vomiting and restlessness in pets.
Larger doses can be fatal.
The lethal dose of theobromine depends on the size of the dog and the type of chocolate. Ounce for ounce, baking chocolate has six to nine times as much theobromine as milk chocolate. Estimates of the smallest amounts that can be fatal. Cats have much different eating habits and seldom are poisoned by chocolate. Cat owners, nonetheless, should keep chocolate well out of reach.