She could possibly have a worm infestation. No matter how well looked after dogs are they can still have worms occasionally.
Certainly do take her to the vet first. Don't use an over-the-counter wormer. More harm comes from using those and can end up with a bigger vet's bill that starting out with a prescription wormer (if that is indeed diagnosed by your vet.)
That can result in stool with a very foul odour. So can Parvo....but she is up to date on her shots, and showing no other signs and symptoms....so that's what makes me wonder about worms.
Coccidia is another possible culprit.
So my opinion is get her checked out -and like Margot said, take a fresh stool sample with you if possible.
Such a little dog, would worry about dehydration. Try feeding small portions of some chicken and rice. If this has been going on this long and has a bad odor, I strongly suggest you get her to the Vet to have her stools checked. Take a fresh sample in with you.