What can cause a late period?

I had my last baby a little over a year ago. I had a tubal ligation when I had my c section. When my periods started back, they were on time every month. Same schedule, never missing a day or never irregular. It’s been 8 days now and no period. I’m freaking out on the inside, but I’m trying to wait it out a few more days before I take a test. Has anyone experienced this or gotten pregnant like this?