Late period

Ever since I had my daughter which 8 months almost 9 months ago . I have been getting my period every month on time , but this month it’s late . 3 days late I’ve been atleast 1 day late . We have been doing natural birth control . Meaning we avoid my ovulation days . I’m getting cramps as if it’s about to come like it always has been doing but now it didn’t . What else could it be ?

We wasn’t trying for another baby until my daughter is 1 and some change .