I'm freaking out

I've had the copper IUD for over 8 months now and in that time my periods have been awful. They're super heavy and generally last 12-16 days with a week of light spotting in between and only 1 week completely period free. The reason I'm freaking out is because I haven't had anything for three weeks now. No period, no spotting in between or tell tale cramps. I've been so tired this past week and I've got this horrible niggle in the back of my mind that maybe I'm pregnant. I know the chances are really low but I don't know why the bleeding has just gone completely awol. I know there's always the possibility that my periods are finally sorting themselves out to a normal length but there was no weaning off at all. I'm only twenty and even though my boyfriend and I are in a relatively good place financially (soon be buying our first house) kids have never been something either of us have wanted, at least not for a long, long time. We're both very career driven. I don't know if I'm getting myself worked up for nothing and maybe I should do a test but I'm honestly terrified of what the answer could be. I guess I just needed to get this off my chest, thanks for reading. Any thoughts would be helpful.