Get Pet Questions Answered by Pet Experts
Because of the breed of puppy you have I would say it is most likely a cyst like structure from trauma. These guys are constantly slamming their chins down while playing.
To be on the safe side...your vet can easily aspirate some cells with a needle...the puppy wont even feel it..but will have to hold still :) !
THe cells can tell your vet what is making up the lump...but I have a feeling it is not a concern.