What’s going on?

So I have the copper IUD. My periods are always on time, maybe a day off here or there. However. Last month my husband did a string check ( we do it monthly). And he couldn’t find them. We’ve had sex a few times and when he’s going really deep he thinks he can feel them. And I trust when he says he’s feeling something different in my vagina. So I haven’t felt the need to go get it checked out and waste everyone’s time.. Anyways. This month I am now 4 days late for my period. No sign that it’s coming. The last couple days I have been getting pretty bad cramping. It comes in waves and then it will go away for hours and then come back again in waves. My period cramps have never done that. In addition to that, I have taken two pregnancy tests after the missed period and they were both obviously negative. What could be going on? Please no rude comments.. just looking for some advice here. Thanks 😊