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.
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.