You don't mention your age or how long you have been menstruating.
It is common for cycles to change for several years after you start menstruation..in some it can take 5 -10 years to settle, some never settle into a regular cycle, some are regular from their 1st period.
Changes in routine (job, school, vacation) can change it. So can changes due to stress (new relationship, break up, financial worries, moving house, death of loved one).
Females living together (sisters, in a dorm, housemates, summer camp) can sometimes be influenced by each others cycle so the menstruate at the same time.