What Causes Bloods Clots During Menstruation?

I have hypothyroidism and taking tab eltroxin 75mcg and 100 mcg alternate days.my last LMP was may 30 th and july 18 to 22 i had only blood spots then it became periods that continued,it was bleeding with blood clots.4 th i consulted with a gynecologist. She advised me to have dydrogesterone 20 mg BID for one week and 10 mg BID for another one week.i completed the course on 17 th aug.during and after the course i still had mild bleeding.my doctor instructed me to go for a scanning once i get my periods,2nd day.how i know that?becoz still have bleeding mild sometimes like periods.i have backpain,once vomited also.what is happening with me?