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If everything else is the same aside frrom what she recently ate then it may be the goose feces . Anything that you pet is not use to eating can cause gastric upset so its possible. I would just make sure she doesn't eat anymore and it should pass...no pun intended. If she continues to have pain after a couple days, goes off her food or starts to vomit or have diarrhea then a vet visit is in order.
The result from ZooCVT seems to be common sense. If your dog has not eaten anything else then it probably is the feces, but I have to say, the response from Elena seems very informative. On some websites you will find information on how goose feces may be hazardous to humans as well as other animals. Dogs can have very sensitive stomachs, especially the smaller breeds. The gastric system can be easily upset when a dog eats any other substance than their usual diet. I agree with ZooCVT, that if the upset continues, a definate visit to the vet is mandatory.
You also might consider the possibility of hip or back problems...I would be leaning to back though because they can come and go...i.e.. pinched nerve..disc. It would explain the intermitent pain and the soft stools. When animals have a back injury, sometimes they will seem tense in the abdomen and because of this pain it affects their stools and sometimes they strain to urinate. If you've ever experienced a back injury, you can definately empthize...I know I can! It can progressively increase if the activity remains the same so if you notice this again, try confining her so she can't jump up and down off furniture or use stairs and leash walk her to potty. do this for about a week then slowly start to reintroduce things but I would try and prevent her from doing any jumping if possible. You can also give her a baby aspirin 1-2x daily on a full stomach only and no longer than a couple days. Should you notice any black tarry stools or vomiting you should discontinue the aspirin...remember..baby aspirin ONLY no Tylenol!.
The goose stool may have just been a coincidence...