app and ovulation sticks

Ali
So the app is off again I was supposed to start my cycle on Tuesday according to the app and I was getting anxiety should I test. Well sat evening and my cycle started that means my ovulation was off for this month I had one ovulation test and I used it on the day I was supposed to be the most fertile in May and it was a negative but my app said I was at my peak time. 
A friend told me to get the strips and test everyday from the day my period ends to find out when I am fertile. My periods are pretty regular I can a month without getting it but that happens only once in a while. We just started two months ago 
I'm 35 soon to be 36 in a few months hubby is 39