Hi there,
I'm probably the last person you would prefer to hear from regarding your fears, but I hope I can put things in perspective for you. :-)
I, too, had a "lump" in my neck that interefered with my swallowing. However, it didn't come and go like yours, but rather seemed to grow steadily over the period of months (as do most thyroid nodules). Mine turned out to be cancerous, and was removed a year and a half ago.
There are two things I'd like you to keep in mind: first of all, thyroid cancer is pretty darn rare. All but one type of thyroid cancer (which is the rarest and ONLY affects the elderly) are extremely slow-growing when compared to other cancers. Secondly, the fact that your nodule popped up so suddenly, has been bothering you for such a short amount of time, and isn't constantly bothersome are all good signs, statistically.
Your best bet is to find a really good, trusted endicrinologist. Be warned: doctors have a tendency to minimalize thyroid cancer. If I had a dime for every time said "Oh, that's the GOOD cancer!", I wouldn't be up to my ears in cancer bills. If the ultrasound is suspicious, don't hesitate to get a biopsy. And if the biopsy is suspicious, get it taken out.
If you need someone to talk to about this, don't hesitate to e-mail me:
spacelovertq@hotmail.com (use the subject "Thyroid").
Good luck and keep us posted!