Usually birth control pills regulate menses. Sometimes the routine menses will decrease or stop completely called ammenorhoea.
This condition wont remain same throughout, sudden breakthrough heavy bleeding may occur.
Heavy bleeding is usually associated with large blood clots
Blood clots of large size is usually confused with fetal parts by concerned women.
Fetal parts have some irregularities when pressed whereas blood clots will be uniform in consistency.
Mostly what you experienced must be a blood clot.
If you have doubt You could do a urine pregnancy
test, it will be positive for sometime even after fetal expulsion
If you have increased cramps, and heavy bleeding it is bettter to consult a Gync.