I would rule out impacted anal glands first.
Excerpts (out of context) from article below: https://www.vetsecure.com/veterinarymedicalclinic.com/articles/136
Overview:
Anal sacs are the reservoirs for the secretions of anal glands which are located on either side of a dog’s anus, at approximately four and eight o’clock. These sacs contain liquid secretions from the anal gland, which, in healthy animals, are normally pale yellow-brown to grayish in color. The contents are usually emptied during normal bowel movements, or when a dog is nervous or scared. In most animals, these sacs empty easily. However, some dogs, especially small breed dogs, are not able to empty the sacs properly and become susceptible to anal sac disease.
Transmission or Cause:
The cause of anal sac disease is unknown. Smaller dog breeds, such as Chihuahuas and poodles, are most often affected. Excessive anal gland production, soft feces or diarrhea, poor muscle tone, and obesity also contribute to higher risk of developing anal sac disease. Anal sac abscess tends to occur after an impacted anal gland has become so severely swollen and infected that the anal sac forms an abscess and ruptures.
Prevention:
Expression of the anal sacs every few weeks or months often will help prevent anal gland fluid from accumulating and becoming thickened again. High fiber diets have been shown to help prevent anal sac disease in at-risk dogs, especially those that are obese.
