Irregular Period

I have had my period since the end of 8th grade — roughly 3-4 years. It has been consistent (getting my period around the 20s of a given month). The past probably 5 months have been so so irregular. For instance, I was supposed to get it over Christmas and it never came. Then January 20s came around and still no period. Somewhere in the teens of February and it finally came. And I never got it during March and it is April 11, and I just got it. Anybody know why? Experience this themselves?