copper IUD and irregular period

Ash
I've had the copper IUD for over a year. I started out with extremely heavy periods, where I have to use ultra tampons and a pad. They only last 4 days also.
But in the past couple months my period had been late by 4 days. Yet on the day it's supposed to start I feel cramps badly for those 4 days.  I still bleed quite a bit just not as much.