Ovulation help

Correct me if I'm wrong, but the ovulation dates on this app are confusing me. I recently stopped taking birth control pills, but am not ready to get pregnant right away. However my periods are sometimes shorter apart and sometimes longer. With that being said most of the time the app would tell me my most fertile days were about a week after my period ended.  Now this month it is saying I am most fertile 2 days after my period ends. Can this be right, it doesn't seem right to me.