Common Reasons for Menstrual Cycle Changes
There are a number of reasons why a woman's period can be late consecutively. However, I am not a doctor so i am basing my hypothesis through my observation with the the women around me (mother, sister, cousins, friends, girlfriend etc) who are constantly throwing around questions and answers as to why they got delayed or why their menstrual cycle suddenly changed. I would strongly suggest to consult a medical specialist in the field or a reliable online source. But there are some observations and conclusions I have made that turned out to be correct. (I have been living with women around me for the best 4/5s of my life).
When women drastically change their eating habits it sometimes affects their menstrual cycle. Not only does food intake change weight it also puts on a great deal of change to the chemical reactions in the brain mainly the hypothalamus which controls hormonal activity (check Wikipedia). Hormonal imbalance can lead to changes to a woman's normal cycle. Medication also greatly affects chemical reactions in the brain (steroids, anti depressants etc) resulting to hormonal imbalance.
Changes in temperature affects menstrual cycles by changing the bodies metabolic rate which leads again to hormonal imbalance. When a person geographically relocates to a place where the climate is warmer/colder the body does not adjust automatically. Women who are used to cold will burn fat faster in a hot climate. This sudden change in temperature may change a woman's menstrual cycle.
Any kind of stress creates chemical imbalances in the hypothalamus (again) affecting hormonal activity. It doesn't matter if it is work, family, school, or your neighbor belting it out in the karaoke machine until morning. Any kind of stress will affect menstrual activity. Stress can lead to eating and sleeping problems (it's a vicious cycle) which eventually lead to hormonal imbalances.
Sexual activity changes hormonal activity when chemicals in the brain (the hypothalamus! you got it!) react to sexual stimuli. Hormonal activity changes a woman's menstrual cycle depending on how much or less sex a woman has. If a woman's sexual activity is constant then the menstrual cycle will also be fairly constant just the same as the lack of it.
We could conclude that most of the reasons why menstrual cycles change is all in the brain. That part of the brain reacts to stimuli and response which changes metabolic rates and hormonal activity which is the reason for sudden changes of a woman's cycle. A woman can have regular menstruation then suddenly become irregular for the common reasons given above but there are many other odd cases in which menstrual cycles suddenly changes.