It could be caused by the change of the number of days in each month! Remember that every month doesnt have 31 days so it can cause your menstrual cycle to come sooner or later than its scheduled ! It can also be caused do to a change in diet ,sleeping or eating habit or stress! Try to relax, get your rest and eat healthy! Keep us updated if anything chances! Peace and love
Sometimes your cycle just changes depending on the girls you hang around (your cycle tends to line up with the women you tend to be near a lot of the time) and sometimes the weather even changes when you get your period. My cycle used to be just like yours (once at the beginning and once at the end) but over time it shifted to me only having one in the middle of the month. It's not really anything to worry about but if you do get bothered by it a lot you could ask a doctor.