As you mentioned that you had previous history of hypothyroidism
there is possibility that there may be recurrence of the same that can cause delayed menstruation
in some cases (though the common symptoms that you suffered previously will be present if there is recurrence) or else as the pregnancy
tests come negative for pregnancy you need to get an ultrasound
done after couple of weeks to further confirm If you have pregnancy or not.Also get checked by a Gynecologist.
with best wishes