Sometimes, a period isn't regular for a variety of reasons. Stress, eating habits, medications...these are the big reasons a cycle may be off. However, some women have a naturally irregular cycle. It's usually nothing to be concerned over, but very annoying.

Has anything drastically changed in your life over the past 11 months?

Have you tried birth control? A lot of women get on birth control because it supplies them with some regularity (not to mention many other things).

There probably isn't a definitive answer as to WHY it's happening. Your body is a tricky thing, and it's annoying!