Less than 1 out of 100 women will get pregnant each year if they use the ParaGard or the Mirena IUD. The IUD is concidered one of the most effect methods of birth control.
According to http://www.fwhc.org/birth-control/iudinfo.htm (A very informative site regarding IUDs from the Feminist Women's Health Center) IUDs are 99.2-99.9% effective as birth control. They do not...
It may be that your IUD has become displaced or fallen out. I suggest you ask for a scan to see what has happened as you may not be protected at all from pregnancy.