Just when you start to feel like you can predict exactly when your period is going to show, everything can change. For that, you can thank the hormone shifts that happen throughout your lifetime.
Once you get your very first period, your cycles may be longer, meaning more time may pass between when one period starts to the next. A typical cycle for a teenage girl may be 21 to 45 days. Over time, they get shorter and more predictable, averaging about 21 to 35 days.
Hormone changes that happen during perimenopause -- the years before menopause when your body starts to make less estrogen -- can throw you for a loop. The time from one period to the next may get shorter or longer, and you may have heavier or lighter bleeding during your period. This phase can last up to 10 years before you start menopause and stop getting your period for good.
Gradual life changes are normal, but sudden, unusual issues like very heavy bleeding or missed periods are not. Talk with your doctor if you notice that something seems off.