Could be several things, depending on your age and particular circumstances: it could be a temporary blip due to stress, or you could be going into the menopause. Of other causes, the most likely is probably fibroids (benign growths in the uterus) or possibly endometriosis, if you have any other symptoms (abdominal or pelvic pain of any sort/bowel problems) or any history of gynae problems.
If it's not painful, you could probably wait a month and see if it's like this next time. If it's worrying you, go to the GP and get checked out - though they may not be able to do much without waiting and seeing if it's just a one-off or it continues.
If you do have pain or other symptoms that continue for any length of time, ask for a referral to a gynaecologist.