I Am 20. Ever Since I First Got My Period

I am 20. Ever since I first got my period back when I was 12, it has been fairly regular, coming about every 30 days or once a month. Back in August it was normal, and then all of a sudden, the next time around it was 50 days later, or 20 days late. I figured it was just something that happened every now and again to women. But then the next time it was 20 days late again. What could be causing that?