My guess would be an abcess, too. I really don´t see what else it could be if it formed that fast. I would ask the vet to investigate further. You don´t want it to spread.
Essentially it must be lanced and drained and flushed several times a day until healed. In some cases a shunt must be used. Encapsulated abscesses can be removed surgically which guarantees they won´t refill.
Antibiotics are advised during this process.
I know some people who have had success with treating smaller abcesses with just Baytril, but there is always the risk infection will spread. That is very bad news.
Pinta and Imenut4u, among others, have had to deal with many abcesses, so I´m sure they can add something to this.