For some women, like you, having irregular periods is normal. The following are reasons you might have a missed or delayed period:
1. Pregnancy.
2. Recent miscarriage or abortion.
3. Loss/gain of 10+ pounds. Affects hormones.
4. Recently-begun exercise program. Affects hormones.
5. Illness. From common colds to ovarian cysts to adrenal disorders, these can affect hormones.
6. Medications. Especially birth control medications, steroids, narcotics, alcohol, and some phenothiazines.
7. STDs.
8. Stress. Even everyday stress can affect those adrenals, which then affect your hormones.
9. Age. Some girls take up to 3 years after menarche (first period) to become regular. Some girls/women never become regular.
Your menstrual cycle is caused by a series of hormonal messages that are sent between your hypothalamus, pituitary gland, and your ovaries http://www.fwhc.org/health/moon.htm. Five different hormones are involved (FSH-RF, FSH, LH, LH-RF, and estrogen) and often the release of one hormone will stimulate the release of another, and so on. This can be a delicate balance in some women, and is easily interrupted.
I'm assuming (of course) that you're sexually active. You don't mention why you don't use birth control, but you may want to consider the different types to avoid future pregnancy scares.
There are conditions (such as thyroid disorders) that can cause irregular periods, but since you don't mention any other symptoms you may not need to worry about them.