Is it common to fall pregnant when you have a copper IUD inserted?

I have had the copper IUD in for almost a year now with no problems. I have always gotten my period every month on the exact day that Im due. HOWEVER my period was due 8 days ago and still nothing. I did a pregnancy test yesterday which was negative but I know that you can get false negative results. Not sure if I should start to panic yet or not. What would you suggest?