You should read the book Taking Charge of Your Fertility. It might help you to understand how a woman's cycle works. A cycle does not have to be 28 days. Some women have shorter or longer cycles. A cycle of 21-34 days is considered normal. Here's an awesome article we have here at MedHelp on your cycles:
http://www.medhelp.org/health_pages/Pregnancy/Menstrual-Cycle-EXPLAINED-by-a-doc/show/515?cid=98
One thing you will learn is that your period doesn't predict when you will ovulate. Ovulation happens entirely separate from your previous period based on a chain or hormonal reactions in your body. It is your ovulation that then determines when your NEXT period will come, usually 2 weeks post ovulation. The time between ovulation and your period is called the luteal phase.
If you can figure out when you are ovulating then not only do you increase your chances of conceiving but you'll also know when to expect your period.
Other things can cause irregularity and one of them would be not ovulating at all, which would then make it impossible to conceive without some sort of intervention.