Typically if it's an allergy it will accompanied by other symptoms like dry flakey skin, dandruff, hot spots, itchy paws etc .... it doesn't sound like that is the case.... Tico was on Timberwolf lamb and apples for 3.5 months, and his poop didn't smell "bad" perse but just strong, strong enough to smell from another room.... I am not sure exactly what is bothering your poor pooch, I can only say what I would do, for me personally, I would look for a product with yucca (which aids in digestion), oatmeal (to settle the tummy), pray I could find one with pumpkin meal (which is phenomenal for tummy as well), and search out a hypo-allergenic or less chance for allergy protein source, like bison, herring or fish (if you can find it where it's naturally preserved).... if the other formulas worked for her I would take back the can of duck, and just switch back to something that worked for her previously.... keep in mind it can take a few days to get it out of her system.
ETA allergies can start with simple gas, of course she could have a tummy bug as well. Take her off the duck and if it doesn't improve talk to the vet and see if it is a bug.... If it isn't a bug and she does fine on her other formulas then presume there is something in the duck formula that doesn't agree with her and avoid those ingredients with future purchases of course
