Your description isn't very complete, especially for how long this lump has been present and whether or not the pup is acting sickly. A lump in that area could be an abscess, an inflamed or blocked salivary gland, a lymph node, or a foreign body. An examination by your veterinarian would be in order at this point.
I had the same problem with my dog two years ago. She was over a year old, but her ailment was called "juvenile cellulitis" which tends to affect puppies around your dog's age. It started with a cut to the muzzle and moved down to the lymph nodes where it swelled substantially over two days. We ended up having to take her to the vet emergency room. Go get this taken care of. It may not be juvenile celluitis, but you should get it checked out.
NOTICE: The information provided on this site is not a substitute for professional medical advice,
diagnosis, or treatment. Never delay or disregard seeking professional medical advice from your
physician or other qualified health provider because of something you have read on Wellsphere.
If you have a medical emergency, call your doctor or 911 immediately.