It does sound like an insulin issue. You may need to work again with your vet on regulating your pet's dosage. SOmetimes there is a shift in how much is needed.
While the sugar in the chocolate might have been right you should know that chocolate is toxic to dogs and should not be given to them.
Better to have used some other source of sweet than chocolate.
You may find these sites useful
http://www.petdiabetes.com/canins~1.htm
and
http://www.caninediabetes.org/
and this one on chocolate and dogs
http://petcaretips.net/chocolate-poisoning-dog.html
No harm done though with such a small amount and the sugar may have saved him!
Hope this helps you!
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