I'd agree Ness - it sounds more like an abcess than anything else. Some of my rats are very prone to abcesses because of where they came from and they do come up very, very quickly and can sometimes grow to an alarming size almost overnight. Antibiotics are great and I usually find that once an abcess bursts and it can be drained etc it heals up very quickly.
I usualy bathe the area with slightly diluted Hibiscrub and put some Vetzyme antiseptic powder on the wound / abcess. I love this stuff as I'm convinced it really speeds up the healing process; the vets I have mentioned to agree that it is good stuff. It can soothe any soreness too - you can get it from bigger pet shops.
Whatever you decide I hope things work out ok. Let us know anyway.