Irregular periods and using this app

I have very irregular periods and after many tests,no one knows why. I have 1 child and have had 1 miscarriage. I can cycle regularly to the day every month, and I could go months, almost a year, without a period at all. I'm on no birth control and this has been the case since I was 11. I'm 29. Currently 12 days late, according to app. Definitely not pregnant. Other than tracking mucus and learning more about my cycle that way, I'm having a harder time pinpointing my cycle on the app. It's guessed accurately a few times though, which I have been very impressed with!