Changes from month to month are also normal. Some months you may have no period, especially during the first year. Your health can make a difference. Too much exercise for example can use up all your body fat . You might not have periods if that happens.
Stress can make a difference, too.
Most girls and women don't feel ovulation when it happens. They don't know for sure when it actually occurs. But ovulation takes place around the middle of each menstrual cycle. Many girls mark a calendar with an X on the days they bleed. For most women periods will happen every 25-30 days.