Hello and welcome to healthcare magic!!!
From the history that you have given of thyroid cancer
it would be better if you got a FNAC done of the swelling because you are at risk of having a metastasis or since the supraclavicular area is the draining area there may be some lymphadenitis
also so a FNAC would give you a clear picture and if it rules out malignancy i think you can be relieved. The low B12 levels are a concern too if they are persistently low despite supplements ask your doctor to test for the intrinsic factor deficiency
along with the FNAC.