Mirena IUD: from no period to the WORST cramping ever

Got the IUD at 21 and stopped getting heavy periods/periods at all (occasional light spotting) for first year and a half. Then started getting my period. And now (for the past year/ a few months after I started getting my period again) get the worst cramps and heaviest (often chunky) periods! Went to Doc, got an ultrasound. All is good. Anyone else have this experience? I feel like it could be endometriosis, but nothing came up on the abdominal or transvaginal ultra sound....