Can Prolonged Heavy Periods With Clots In A Long-term Hypothyroid Patient Indicate A Serious Problem?

Hello i have underactive thyroid for 7 years and have had irregular periods, iv not had periods for 5-7 months but this june i had my periods and they havent stopped their heavy with clots, doctor has given me noresthisterone but after i stop periods start again and heavy and carry on! Now iv been on noresthisterone for a month so period is stopped and i have a transvaginal ultra sound next week! Im scared if it could be something serious?