What Causes Heavy Periods With Clotting During Fertility Treatment?

I am currently under fertility treatment and on my first cycle of Clomid, I took Norethisterone for 10 days and my period arrived 5 days later. I am worried as it is very heavy with clots, i have now been flooding through sanitary towels in 30mins - 1hr for 4 days now. is this normal and will it stop the clomid doing its job? thanks