Lots of things can cause swollen lymph nodes, so you should definitely try to visit the doctor as soon as you can. At the very least they should check to see if you have some kind of infection, and also make sure that your white blood cell count is normal. That being said, I've been recently diagnosed as hypothyroid and have been having swollen lymph in the back of my neck (as you describe) and around the sides for the past 3 months or so - they are painless, though.
I can only share with you my own experience. I just had a total thyroidectomy because one small node (among others) had grown just slightly, after ten years of watching via ultrasound. Just during the last year, I have had swollen lymph nodes in the back of my head and neck, which were also seen on ultrasound and were deemed "normal." One was biopsied and came back negative. All I can say, is that since my surgery, the lymph nodes have not been swollen. Coincidence? I don't think so. I also had headaches in the back of my neck and most of the time had trouble moving my head side to side (not too much range of motion). This has improved as well. If you are waking up in excrutiating pain, go to the Emergency Room or to a doctor. I realize you don't have insurance, but something should be worked out. Three weeks is a long time for these symptoms.Good luck!