If you've been following the tales on this site for a while, you will know that we strongly recommend that you do not consult 'Dr Google'. Much of his information is out of date, poorly researched or hones in on spectacular cases. It leaves us all thinking that we are facing the worst case scenarios. Fortunately, this is seldom the case.
I am glad to hear that your ultrasound showed that your lymph nodes were fine. Have you had a cold or any infection recently? This is all it takes for your lymph nodes to swell. I am not a doctor and do not know a lot about thyroid cancer, as I have had 2 bouts of breast cancer, but I would have expected your ultrasound to show if there had been any spread.
You are way ahead of yourself thinking of it mutating to drugs and causing anaplastic thyroid cancer. I hope that your biopsy goes well on Monday. It will probably take a couple of weeks after that before you get your results. This is a scary time, but we all get through it. I hope that there is a simpler explanation for your lump.
What ages are your children? You will obviously be worried about them, but I'm sure that you will come through this a stronger person.
Please let us know how you get on and remember that we are always here for you.