Swollen Lymph node

Hi,  My lil jack Russell is five years old this month.  He has recently become less enthusiastic about eating and does not want to play.  He will run in the yard but refuses to pick up a toy in his mouth.  I took him back to the vet today due to continuous behavior of seeming fearful, and clingy.  Today they found a swollen lymph node under his jaw.  The vet said it could be cancer or due to a problem with his tooth.  The same side of the swollen lymph node he does have an upper brown tooth.  Could this cause the swollen lymph node?   I am so worried about cancer now that it has been thrown in the mix.  She is going to clean his teeth and examine them further tomorrow and do a biopsy on the node.   I just cant fathom the idea that my five year old puppy could have cancer based on one lymph node and a higher wbc.  Couldnt a tooth problem explain all of the above?  
Thanks so much for your time.

[Justine Lee, D.V.M., DACVECC]

It certainly could all be due to a tooth root abscess or infection. I know it's nerve wracking, but I would wait for the lymph node aspirate or biopsy (make sure it's submitted for analysis to an outside laboratory). That said, get the procedure done as soon as possible! Keep us posted and we'll cross our fingers that it's a tooth root problem!