Cramps??

Hallie
So I got Skyla IUD almost a week ago. It'll be a week this Tuesday and that day (my boyfriend and I) have been having unprotected sex, but he also has been cumming inside of me..
I've been having weird cramps I've never gotten before/never felt before. My period isn't due till 25th. My fertile window is this week. Anyone explain to me why I'm having random cramps? 
I took a pregnancy test and it said I'm not pregnant.