Fertile days after period?

Tanya
Hello ladies,
​So I have seen the pictures that some of you have uploaded here where I can see your <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period calendar</a>. Why in some of yours your fertile days starts right away after your period? Mine says I'm fertile about a week after my period. I'm a little confused in how this is set up.  
​Thanks!