I am sorry you're dealing with new health problems. I am not a doctor, but it is my understanding from literature, that it is possible that your cancer has metastasized to the thyroid. It is also possible, maybe probable, that it is an entirely new unrelated problem. I would think they will do a biopsy to determine what's going on with those masses. Those samples are sent to a lab, then your doctor will get a pathology report that should give them more information to be able to see what's going on in there. It's my understanding that it's rare to have a metastasis to the thyroid, but it cannot be ruled out.
In the case of my dad, he started out with lung cancer. Two years later, a mass was detected in his liver. So, of course we thought that must be the lung cancer popping back up. Even the oncologist thought that. It turned out to be an entirely different cancer. I learned from that never to make assumptions, and to wait for the test results. That's always the fun part-- waiting. Best wishes to you.