Allergic reaction, trauma, tooth root abscess, and infection are common causes of facial swelling. If your dog has any fractured teeth in the area under the swelling it could be a tooth root abscess--most infected teeth do not have pus draining and many do not have gingival (gum tissue) recession. Peri-ocular, muzzle, and lip swelling is common with allergic reactions.
An uncommon cause of pain when opening the mouth is a condition called masticatory myositis (inflammation of the muscles that enable chewing).
Your dog needs to be rechecked by a veterinarian. If no other causes are noted on examination then he needs to be sedated for a thorough oral exam, probing of the teeth, dental and/or skull x-rays, and a biopsy if appropriate.
An uncommon cause of pain when opening the mouth is a condition called masticatory myositis (inflammation of the muscles that enable chewing).
Your dog needs to be rechecked by a veterinarian. If no other causes are noted on examination then he needs to be sedated for a thorough oral exam, probing of the teeth, dental and/or skull x-rays, and a biopsy if appropriate.