There is no point just being worried about this. Go get the lump checked out. I advise you to do it before the holiday period, as if suddenly needed, vet offices could be closed then for days....and worrying about it could spoil your Christmas. It is better to find out what this is, and deal with it straight away.
I can't tell you what this lump is. It could be any number of things. The vet will probably have a pretty good idea, but to be certain, may recommend doing a needle biopsy, which is fairly quick, and doesn't need anesthetic. Blood tests may be done as well, or other diagnostics if needed.
This could be ....a lipoma, an abscess, a cyst, swelling of the gland for some reason (very often neck glands swell when there are problems with the teeth or ears, or even a small local abrasion, cut, sting or insect bite)....the larger percentage of lumps found happen to be benign, statistically. But of course there IS always the possibility this could be cancer, and if so, it would need to be dealt with quickly.
So make an appointment to see the vet.
Thanks, I have book him in today for Tuesday, as my same through was it is too close to the holidays.