period late on IUD... help

So I have had the skyla IUD since May, so almost 6 months. My period was supposed to start on Saturday but it didn't (or at least I don't think it did). On Saturday I woke up with some very dark brown discharge and I assumed I would start my period but I didn't. Instead over the course of Saturday -  Sunday it just turned to a very pale brown discharge that wasn't heavy, it's really only there when I wipe... should I be worried? I am sexually active but my boyfriend and I are very careful and always use condoms. Just wondering if anyone else has experienced this before... I'm freaking out about not getting a " true period"