This is understandable that you are nervous. I actually didn't get my period for a full year after I gave birth due to breast feeding. No birth control used! AND you have an IUD which often will cause this too. I got my period after I stopped nursing actually during the weaning process. Is your IUD hormonal? If so, this would be pretty typical. You can always check in with your doctor about this too.
If you took 5 pregnancy tests, you certainly aren't pregnant, so probably your body is still adjusting to having had a baby. If you're breast feeding that could also help explain why your periods are AWOL.