I heard something along the lines of "cancer isn't on and off, it's on and worse". So the fact that your symptoms seem to come and go is promising. Also, if your doctor was incredibly worried about your symptoms they would have sent you in very quickly. Don't forget, these folks are professionals and have seen many problems in far more detail before.
As far as the lymph nodes and thyroid, I think both of these are pretty sensitive and can become inflamed from being messed around with too much. My biggest recommendation is to try to avoid fiddling with them and at the most try something like a warm/cold compress to see if that helps relieve any discomfort.
Additionally, I know one thing some singers can suffer from are vocal nodules, but I think that's more frequently seen in women. Being a singer you put a lot of strain on your voice, I know mine gets sore for a day or two after a long car ride full of singing. I'd imagine there's a higher likelihood of it being connected to that than cancer, especially with your age and lack of tobacco use.
You're doing the right thing by getting things checked out, but worrying can only exacerbate the problem. Good luck and keep us posted