Are you taking or have you taken any birth control? That can effect your cycle. Stress, unhealthy eating, and even infections or illness can throw off your cycle. Hormone imbalance or hormonal changes can also effect your cycle. Of course, the only way to be certain what is going on is to evaluate these factors in your life and get a good exam from a gynecologist. It sounds like you are too young for menopause, but there is something called perimenopause and early menopause which could be possible. The only way to know if you are experiencing those things is to take a "Hormone Panel". That will test all the levels of your hormones.