A big question

Tee

July i bleed only for day 13th to the 16th right my next peeiod is expected aug 20th i stared bleeding aug 9 th to 12th i downloaded app told me i will be expected my next period tommorrow how

My dates for period

May 5th lasted 7 days

June 5th lasted 7 day

July 13 th lasted 4 days

What am i missing