Havent started my period but im not pregnant?

I need help!! This month I have not started my period,but I am not pregnant. I've checked a bunch of times with so many pregnancy tests. Also I cannot be pregnant because I have been on the paragard iud since April 2018 which was 3 months after I had my son. Since then my period has come every 20+ days. NEVER FAILS. Well this month I was supposed to start around the days 5-7 of December. It's now the 13th and according to glow I'm 6 days late. Last month I started around the day 4th of November and my periods are 6-7 days long. Can anybody give me some insight on what this can be? Also I've been getting cramps on and off like I'm going to start and then nothing! Im freaking out. If it helps im 24 with 2 kids