What Causes Heavy Bleeding & Clotting During Menstruation?

Hii.. I have hypothyroidism and due to this i alwayz have mesnustrual problems regarding prolong bleeding..but this time i had my ususal periods and just after four days it started again and heavy with clotting ..i went to the doctor after seven dayz ..she prescribed me sonulet .. As i searched abt this medicine this will cause bleeding again after discountinuing this medicine .. So is it right to take this medicine ?