Trying for baby #1! I have questions!

Cher
My husband and I just decided to start trying last month. I have always had heavy, long (like 10 days) periods. I have no reason to believe I don't ovulate (as of now) but my period is so long according to this app my highest fertility chance is 18.4 the day after my period ends. This seems very low? My dr has never showed concern or anything.