This is one of the major reasons to choose a thyroidectomy as Graves patients have a higher risk for papillary thyroid cancer. You are fortunate you did as the cancer would not have been discovered. As they removed the thyroid and the nodes, you are in good shape. Thyroid cancer is a very curable cancer, especially as yours was caught before any symptoms. They sometimes recommend radioactive iodine after Graves thyroidectomy in patients with no cancer just to be sure all of the thyroid tissue was removed so if this is recommended, it is just to be sure all tissue is gone. Everyone is different, but it usually does not take long to find a good thyroid hormone replacement dose. You should be feeling much better soon. In my case, it was a matter of a couple of weeks.
So glad for you that you chose a thyroidectomy and the cancer was found and removed.