misdee you can have delayed af for any number of reasons.
If you've been run down that could delay ovulation which in turn would delay af.
sometimes people skip periods for no apparent reason and they return a month later
you could have had an anovulatory cycle due to illness and consequently af may not have turned up.
If you've radically changed your eating habbits/exercise regime over the past month that could have an impact.
I skipped a period nearly two years ago and I was actively ttc so convinced I was pregnant despite neg tests. Everyone on here told me I was pregnant, gp told me I was probably pregnant, I even had pregnancy simptoms and wanted to tell people I was pregnant. And mmy af arrived exactly 28 days later .
Yes it's possible to get a negative test and be pregnant, but it's more likely to have a negative test and not be pregnant.
If you haven't had a contraceptive failure then it's unlikely you are.