This is not one of the resons for which you should panic, 30% o all women are in the same situation as you are. Still, it is good to go and see a doctor, just in case... Below are the most common reasons for your issue. You can read more on http://www.womenhealthzone.com/womens-reproductive-health/menstrual-cycle/reasons-behind-irregular-periods/
Significant weight gain or weight loss is one of the reasons for irregular periods. Even though low body weight is the most common cause of irregular periods, obesity (obesity effects)also causes various interruptions in your regular menstrual cycle.
If you are a victim of severe emotional stress, then you can possibly experience irregular or missed periods. This is due to the reason that stress will have very serious impact on your reproductive health. So, if at all you experience severe emotional stress, you can obviously suffer with irregular periods.
Various eating disorders such as anorexia or bulimia nervosa can also have influence on your regular menstrual cycle. So, if you have any kind of eating disorder, you can certainly experience irregular periods.
Excessive exercising or straining your body also leads to irregular menstrual cycle. This is the reason why most of athletes encounter missed periods.
Many new mothers do not resume their regular periods until they have completed their breast feeding. So, they can possibly have irregular menstrual cycle.
Consuming excessive alcohol can also disturb your hormonal metabolism. As a result, you’ll certainly end up with irregular or missed periods.
Certain uterine abnormalities such as cervical polyps, uterine fibroids and endometriosis (endometriosis symptoms) also lead to irregular periods."
You might want to get tested for thyroid disorders too if you haven't before or if it runs in your family. Thyroid problems throw off your cycle big time.