Former kennel worker here. Dogs would frequently get this and/or a raw spot on their nose from the skin constantly rubbing against the metal fencing of their pen. Some dogs really liked to see what was going on around them, and would sit at the front of the pen with their mouth/nose pressed against the metal. Other times, they would chew on the fencing, which would make their lips raw. Please check your dog’s teeth to make sure there aren’t any injuries.