From what I've been able to understand about intestinal infections, it can be common for them to change in size and shape from just about one day to the next. Maybe this has something to do with gas produced from the infection, or just plain inflamation from it, I really don't know. I still think another dose or two of Flagyl would help out immensely here to make absolutely SURE!!! the infection, and/or what is CAUSING the infection is TOTALLY eradicated, not just subsided some. Maybe even a couple doses of a good reptile antibiotic "Amikacin", but not at the same time as giving the Flagyl, that might be a little much for the young snake.
What if another swelling occurs sometime soon, are you going to give it to the vet as you mentioned before?.........He probably gave it back to you early(4-5 days) as you previously mentioned just in case it didn't pull through, then it wouldn't have happened while at his facility, and he might think it would "look" a little better for him in this situation, but of course I'm not saying that is absolutely the casehere, just a possibility. But it will NEVER make good sense to me that a snake with THAT BAD an infection, could be expected to be totally okay afterwards with only ONE Flagyl treatment???, even much lesser cases are usually given two doses, in this case I would use 50 mg/kg body weight(repeat in 14 days).
Either way, I really do hope the snake stays okay, and has a long healthy life. But if it should show ANY MORE signs whatsoever of this thing re-occurring, it really needs to see the vet again, either for more antibiotics to finish-off the infectious pathogens for good that were there from before, or just plain given to the vet so he can "HOPEFULLY" treat it as needed for better odds of it surviving at ALL.
Again, I hope things go well for the little one.
Keep us posted about this still, and give what I mentioned some more thought.
take care, ~Doug
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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"