Hi Kimberly and merry Christmas,
I am sorry Shimmer has this trouble.
It is either gingivitis, an abscessed tooth, oral sore or resorptive lesion.
For more information on Feline Resorptive lesions, please visit the following links:
Some of these things are not visible to the naked eye however you should take a look in case she has a stuck item
Check under tongue and roof of mouth.
She is not in immediate danger as long as she is still eating. It is a good idea to pick up any dry food and feed smooth pate canned until she can be seen after the holiday.
Please let me know how she is doing. Reply here or if you prefer we can speak on the phone.
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