yeah so freaking out

i got my first period in January when I was 11 months post-partum and I expected it to be regular after that, since it was regular after my first baby. according to my usual cycle, it's due today, but no sign of it. i had sex once after charting and believing I was post ovulatory plus a few extra days to make sure I was very likely infertile. obviously I could test, but even if it's negative I know I'm gonna keep worrying. I'm not ready for another baby. i planned to be done with kids. how likely is it to be irregular even after my first period?