Hi, lumps on the vaccine site are quite common. Just watch to see if it gets any bigger. They can take a few days to quite a bit longer to go down. Hope it does. It's the aluminium they put in the adjuvant of the vaccine which causes the reaction. If it isn't going, your vet may want to do a biopsy and scan, but that's a way off - I'm sure he'll wait to see if it goes down first cos most do.