What happened to my duck?
Assuming you didn't do some kind of "flavor injection" treatment, there is absolutely no reason why one part of a bird would taste radically different from another. I'm going to go with "Bad Duck."

Cooking the bird slowly seriously reduces the possibility of a "hot spot" causing some kind of local taste variance. Breast meat will become overdone at the point where dark meat is still tasty, but that wouldn't explain smell or taste, and unpleasant smells are almost always related to bacterial issues.

If it was discolored, it could have been local bruising, which would cause some taste issues, but, again, wouldn't explain the smell.