My period in july was late 3+ days. it started on july 22nd. being late caused my average to go from 29 days to 32-36 days. im starting birth control but cant until my next period when should i expect my next period. how do i know when i ovulated?

Menstrual cycles can fluctuate from month to month depending on hormone fluctuation caused by stress, weight loss/gain, exercise, and lack of sleep. It is not usually a concern if this happens once or twice. Ovulation dates also fluctuate based on hormonal fluctuations. Use condoms/other contraceptives until you have been on your new birth control for 10 days at least.