D.H.
Most likely if it's been there for a long time, it's just that, a lymph node that's swollen. But I can understand why your mom is concerned. I had thyroid cancer, and the first thing I noticed was a lymph node that had swollen up with an infection and then didn't go away after multiple types of antibiotics. One specialist that I saw thought it was a brachial cleft cyst, which is apparently fairly common. He explained that they tend to be there, worsen with illness, then usually go away, until one time they just don't go away. So that could be what it is. The symptoms your doctor has you watching for sound thyroid related (tiredness, etc.)
I think that the only other way to test to see what it is would be a needle biopsy...probably not what you would want your daughter to go through. Maybe a CT scan or an ultrasound may help, but those aren't very conclusive.
Good luck!
D.