Hello Mo,
Have you had your thyroid hormone levels tested since you have been on the methimazole? Where are your thyroid hormone levels now?
The thyroid both makes thyroid hormone for the body’s use and stores thyroid hormone. Some of the thyroid hormone the thyroid makes goes into storage. The body must get rid of the stored hormone before the thyroid hormone levels begin to drop. That can take several weeks. Are you experiencing the other symptoms of Graves’ disease? If you are it may mean that your thyroid hormone levels are still above normal.
Maybe your metabolism has been set to losing weight. Does it automatically reset itself to gain weight when your thyroid production slows down?