You should first start getting regular periods. So many girls at the age of 19 years may not have established regular periods. Once you get regular periods ( at least 6 months of regularity as far as amount of blood loss, duration of flow and interval between two periods) , then any deviation from anyone of these factors will be considered as irregular periods.

When it doesn’t follow a regular pattern

A normal menstrual cycle is the gap between 2 day one of periods is 21 to 37 day Bleeding including spotting during periods should not exceed 7 days Beyond this it's an irregular cycle