BFN but no period


So, my period is never late for more than a year.

According to the app, my ovulation was on 12th aug, was suppose to get my period on 27th not nothing yet. Had cramping and back pain for past 2 days and I feel little tight in my tummy area... took a test again today but BFN, I think I am out this month, but what could other reason be...? Any thoughts please!!