How did it go at the vet?
Post neuter abscess would be my pretty solid guess. These can happen months post neuter...an infection can brew for quite some time before any clinical symptoms (i.e. the swelling) occur.
Whether antibiotics are essential is up for debate, I have had three pigs recover from abscesses without antibiotics (one was on Baytril for a time, which did absolutely nothing, he eventually recovered from the abscess having not been on oral ABs for some weeks). One was also a post neuter abscess.
I have heard of pigs right at the other end of the scale with these abscesses, I recall a the boar of a member here, lovelygirl. Her pig was treated by Vedra for this abscess, and antibiotic beads were eventually stitched into the cavity in an attempt to heal it. The boar did recover.
You will need to keep the abscess open, in my experience I found it easier to do so when the abscess burst by itself but have got the hang of the incisions as well now. You need to squeeze the abscess from all angles, particularly the point lowest the ground (where the pus mainly collects) at least 2-3 times per day. I tend to keep going with the regular squeezing for at least three days after it seems "clear" - and by clear I mean either nothing comes out at all, or no pus but a little bit of blood.
If the abscess does turn out to be more stubborn, flushing the cavity with the antibiotic Flagyl (metronidazole) is one treatment approach.
Good luck!
