Oral antibiotics are pretty useless with Pythons because their metabolism is so slow. I've seen this a couple of times over the years. My thoughts would be a broken tooth that got impacted in the gum line and thus infected or a small scratch inside the mouth from a rodent nail (even FT) and caused an infection. At this point the area may need to be drained and debrided coupled with a round of injection antibiotics. If its an impacted tooth the tooth that is causing this will need to be realigned with the proper place in the gums after the swelling and infection is drained.