Period and fertile

Judit
Hello everone
I need the help
I used glow from march, because id like baby. Easy program and help remember my period and fertile ;) i set cycle length and period length. 
I have a system now, when come my period. Always every 23th day. But glow think not. Very important the period because fertile. My problem: 
I check my phone, i see my fertile start, huhuuuu ok, about half term go to bed ;) nothing happened, after one week my period came, anyway, I set in glow, and the program change my previous month too( when was my fertile) 
So, i not understand why change my previous month?
I can not be based on the program, need to be try ovulation test ;) 
I hope i was understandable, because my english not the best ;)
Thank you ladies!