AF -.-

Thought I was pregnant 
Felt all the symptoms 
But got AF Sunday..
Am I doing something wrong? 
I am married 19 year old & have only been with one person ( my husband)
I have been using this app to keep track of when I am fertile and I'm wondering if I did something wrong
Or if it's something wrong with me. 
Really trying for my next cycle (would be w July 31 due date) 
Advise ?!