Sounds like an abscess, I have only dealt with two piggies with abscesses, one sadly passed as I have learned now I think the antibiotics were not strong enough and he should have had it lanced or removed sooner. The other one, my vet took a more aggressive approach with stronger antibiotics and surgery and I flushed it twice a day at home for 10 days. Good luck with everything!
Oh and your piggy is adorable, very unique looking! thank you my vets are lovely and want to see him again monday so i will say ro habe it drained if its still there i adore him sooo much ive had piggies all my life but hes the most friendly loving little boy, i wont ever forgive myself for gettin him done if anything happends to him x