This is not one of my favorite topics, but it can take on a high degree of urgency in some households.
Dogs have a pair of anal glands (also called scent glands) on either side of their anus that secrete an intensely foul-smelling fluid that marks their feces. After they have a bowel movement, every dog within several miles will know your dog has made a deposit at that spot.
These glands can become partially or completely blocked, often for unknown reasons. As a result, there may be some degree of straining and a larger quantity of "canine aroma therapy" is excreted. Dogs that are having problems with anal glands will often scoot across the floor, or you will notice a very strong, unpleasant odor emanating from their rear end. They often will incessantly lick at their rear end, which becomes even more disturbing when they then lick your face.
Your veterinarian or groomer can express (empty) these glands on a regular basis, usually every one to three months to prevent problems from arising. Sometimes veterinarians will infuse these glands with a medication to prevent infection and inflammation.
Armed with latex gloves and KY Jelly, some highly motivated dog owners learn how to express their dog's anal glands themselves.
In some dogs, the glands can become totally blocked over a period of time; the condition progresses to an infection, and finally a rupture and abscess that must be surgically treated.
For this reason, I would recommend an initial visit to your veterinarian to have the glands evaluated and expressed, and to discuss the topic further.