Late periods/not due to pregnancy

What are some reasons your period would be late other than pregnancy?

My period has always been on time, and even after birth, it fell right back into the same cycle of 28-30 days. Now I’m almost a week late. And it’s been feeling like it’s going to start for a while.. but nothing. After my last c-section, both tubes were removed.

Just wondering what everyone’s experiences are with late periods. At what point do you worry about it being late?

Thanks!