Hi Canadalover. Welcome to the forum.

I should start out by saying that I'm not a doctor.

You would not be human if you were not worried, but there are numerous reports on this site from people who've had long term swollen lymph nodes which were eventually proven to be harmless reactions to infection.  Nodes can and do remain swollen for months, long after the infection that caused them to swell has been handled by the immune system. That said, you were right to raise this with your doctor, and the doctor was right to send you for an ultrasound, but that does not mean that cancer is a done deal - far from it.  I know it's easy for me to say try not to worry, but nevertheless....

You ask whether the lymph node can be a result of the root canal.  I'm no expert on dentistry, but on very rare occasions a root canal can go wrong. Perhaps a piece of file breaks off and can't be removed (but your dentist would have told you if that were the case) or despite the dentist's best attentions, some infection creeps into the tooth where it continues to fester.  I think if I were in your situation, I would wait for the result of lymph node to come back, and if it's all clear (fingers crossed) then go back to the dentist and ask for a X-ray on the tooth to see whether the root filling is stable or is changing. 

Please tell us how you get on.