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With your description, I would be concerned about an anal gland issue. The anal sacs can become full of fluid and can be irritating causing dogs to bite, lick and scoot their rear ends on the carpet. These can easily be expressed by a veterinarian or groomer. Other causes of biting at the rear end would include fleas or possibly an insect bite (you should be using monthly flea prevention, even if you do not see them). There are other reasons that she may be uncomfortable but this would require a veterinary exam to determine if any of these are going on as they are less common.
If she is still doing this in the morning, you can take her to a groomer or your vet and have her anal sacs expressed (most places will do this as a technician exam). If this does not help then I recommend a vet exam to determine if she needs something more to address the discomfort.
Does this address your concern today? Please let me know if you have any further questions, I am happy to help you if you need further assistance.