It does sound like it could possibly be a tooth infection/abscess. If it gets really painful your dog will probably stop eating. But often dogs with tooth problems keep on eating anyway, unless it's really bad. I'd eliminate that possibility first.
Is it easy for you to examine your dog's mouth? Sounds like it would be on the bottom jaw. You might see something. Or get the vet to check him out. If it is this, antibiotics will likely sort it.
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