It would help if you can post a pic or pics of this. He just ate, so it might be that his mouth didn't close correctly- sometimes they catch one or more teeth on their own mouth, & part of their 'lip' gets folded under (inward). That's why you always see snakes yawning & stretching their jaws- to straighten their rows of teeth. You might need to help him with this, if that's what it is & it continues (if he's unable to fix it himself).
Do you feed live? If so, could be a rodent bite or other injury during striking at prey. A persistent swelling that results from a bite or other infection can indicate an abscess in a snake, & because snakes tend to have solid pus, will NOT drain without surgical help from a vet.
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