There are multiple possibilities. It could be caused by a trauma of some sort. For example, if your dog likes to get into the trash or chew sticks or other hard objects she finds laying around, she might have injured her mouth that way. If she plays with other dogs and it's near her upper jaw and she has excess skin in that area, that could have gotten folded over her upper teeth when she grabbed another another animal and she bit herself. Or it could be caused by an infection or be a growth of some sort.
I agree with Doc Myrna about the likelihood of some type of foreign body or trauma causing this wound. Sometimes the foreign body will become imbedded in the tissues and needs to be removed before any healing can occur. If the foreign is already gone, the tissue will usually heal pretty quickly once the dog is put on antibiotics. Alot of this answer depends on other items...how old is your dog? A senior dog might have a cancerous growth in that area, for instance. It also could be related to a broken and/or infected cheek tooth. A thorough exam, preferably under sedation should steer you in the right direction.
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