Not knowing your age or circumstances it's not easy to answer your question. Then again your question may not be personal. Here are details I hunted via google that may help you. As you can see there are many reasons why a menstrual cycle varies.
- New cycle , that is the irregularity experienced by some young girls at the onset of the periods. Their cycle can be irregular for months or years whilst it is establishing itself.
- Dietary problems , eg anorexia, bulimia or simply poor diet
- Too much exercise
- Drugs - prescription and recreational
- Break in routine
- Thoughts and emotions
- Sexual activity - especially when it is a new occurrence
- Anxiety about pregnancy
- Illness or physiological imbalance eg thyroid conditions
Irregularity also falls into 2 categories:
- Long Term Irregularity
Long term irregularity can be anything from a cycle that varies in length from month to month to the experience of various abnormal symptoms, for example, excessive bleeding, no cycle for months at a time, very painful periods or ovulation.
To be on the safe side if your are experiencing prolonged bleeding or very painful symptoms it is wise to consult a doctor to find out what is going on.
- Sudden or Short term Irregularity
Short term irregularity can be attributed to any of the causes listed above, and are generally a one off or sporadic occurrence. These cases are probably not a cause for great concern unless you are experiencing pain or just feel as if something is wrong.
Naturally, a delayed period can indicate pregnancy so you may want to clear that option by taking a pregnancy test.