Welcome back to Heathcare Magic Physician's Desk!
It can be an enlarged lymph node
due to inflammation at the lip pierced site.
In that case it will subside when the lip wound heals.
But more likely it is a furuncle- a boil filled with pus.
In both the cases you need a course of antibiotics like Augmentin
for 5days and local drainage
of the boil if it is there in supervision of a local physician.
Wish you a great health!