Sounds like the dog was sensitive to the drug (a morphine derivative). Because every individual has a unique body chemistry, even though doses are given "mg/kg" carefully calculated to the dogs weight, they can still be more or less sensitive to the drug. In the people ER, it's called "narcotic naive". It will wear off, and if she ever needs that drug again for moderate or mild pain, start with 1/2 strength. If the pain is severe, though, don't use half strength.
There is no way anyone knows what ones reaction will be to a med if it's never been given to that dog or person before.