Sometimes periods can altered by a change in diet, change in weather, and change in exercise. Another reason can be you syncing up with another friend's cycle. Periods aren't as regular as people make them out to be. As long as you are healthy you shouldn't worry, if you are very concerned you can visit a doctor.
There are several things that can cause your period to move around in the month. One is a change in diet or exercise. If you are losing or gaining a significant amount of weight, that could affect when you get your period. Another potential cause is if you are living with other women. Perhaps your cycles are regulating themselves together.
Periods will fluctuate in length, even in a normal healthy woman. I have found for my own body chemistry, high fat diets have caused my periods to be more frequent, heavier or longer. This is consistent with the opposite effect, where extremely low fat diets cause periods to disappear.
Depending on your age, you could be entering Menopause - this phase is called Perimenopause. If you are younger it is possibile you have endometriosis. This is where the cell from the uterine lining somehow grow outside the uterus and attach to other organs. This causes excessive bleeding.
If you have changed birth control, from mixed pill to mini pill for example, you may see your period become longer and therefore they would fall closer together every month.