This sounds suspicious for an anal gland impaction. These are the doggy equivalent to the skunk spray glands, and small breed dogs can have problems with them sometimes. This might be the first time it has ever been an issue, and it might never be a problem again.
I recommend taking her to a groomer or your regular vet to have her anal glands expressed. It is very quick, and should resolve the problem immediately.
If the problem recurs, you can try adding fiber to her diet to naturally express the anal glands every time she defecates. Some dogs simply require regular expression (I have one patient that I see every 2 weeks for this!).
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