Hi M.,
I have been in a similar situation. I have had PCOS (Poly Cystic Ovarian Syndrome)for a number of years. My periods are very irregular, and have been since I was about 25. I am now 40, and although the doctors tell me that I am too young to be going through menopause, I know that I am because it runs early in our family. I also have hot flashes and night sweats. However, I doubt if that is a problem with you, although it could be. I would schedule a complete physical with a gynocologist, and be sure to tell her that you are having this problem. She can prescribe primarin for you. It is a form of progesterone, the hormone the body produces when it is not pregnant. You only take it 12 days a month, and that is only when you have not had a period for 40 or so days. I have used it before, but now I just don't bother. I had my tubes tied when my youngest daughter (now 17) was a baby, so I don't worry about pregnancy. Your gynocologist may also want to do a laproscopy or other tests to make sure you don't have endometriosis. And as several of the other ladies mention, it could be caused by stress.
I hope this helps, and I hope that when your doctor digs a little deeper, it turns out to be nothing much.