Do you think the lump is hard or does it fill like its filled with fluid?
Could be a hematoma , but it would be the best idea to take your puppy do the veterinarian.
Better safe than sorry.
It would be very difficult to say what this lump is without seeing it, and feeling it. Does your pup wince when you touch the lump, or is it reddened and looks inflamed? If so it could be an abscess.
It could also be a swollen lymph node.
I obviously can't say for sure, but if it is very rounded and smooth, especially if it seems 'mobile' (moves quite freely under the skin) it sounds like a cyst, and is most likely benign.
However please don't take my word for it, as some cysts are problematic, and also,a small percentage CAN turn cancerous later if left.
Always the best idea....get the lump checked out with the vet.
My dog has a number of lumps and little cysts etc, which she has had for a while. I always keep an eye on them as the vet asked me to do. None have grown, some have disappeared! But every time I did discover a new lump on her I took her straight away for it to be investigated just to be on the safe side.