The length of your "cycle", meaning the time between the beginning of one period to the beginning of the next period, can be affected by physical stress due to excessive exercise, psychological stress, and illness. Most of the time an era of irregularity is the body's normal response to events or issues that will pass and then regularity returns. No big deal!
If you consistently have a cycle that is longer than 38 days and irregularity, then the underlying cause could put you at risk for long term health problems that need to be treated.
What is the cause of long and irregular cycles?