It is possible that some thyroid tissue remain after thyroidectomy
especially as you have fluctuating levels of hormones indicating probably that your body might be producing some hormones. To support this, as you pointed out, is the lump you now feel in you neck. So you may be having a recurrence of goitre
(enlargement of thyroid tissue. With respected to when do you see you doctor, i would advise you stay and wait for your next appointment; however, you need to contact her immediately i case of any other abnormal feeling or worsening symptoms.
Hope this help,