Concerned

So according to glow I'm 15 days late. I have had my copper IUD for 4 years now and I'm quite regular with my cycles. I bled a small amount for a little bit 1 day 3 days before my period was supposed to come and have had no signs of a period since then. I've taken 4 tests all negative within these last 2 weeks and I'm getting a little concerned. Has this happened to anyone else? I've had pregnancy symptoms like I did with my 1st pregnancy but I'm at a loss really.