It could be that you are intolerance to IUD, in which case bleeding will have to stop and pain subside soon. However, the most important thing to role out immediately is extra uterine pregnancy
which occur in rare cases. For this you will need to do a pregnancy test (and ultrasound
if pain persist). If no pregnancy and bleed still heavy then dysfunctional uterine bleeding
is possibl,e in which case hormonal
treatment (by doctor) will remedy the situation. So has pregnancy test done.
Wish you good health,