Hi, how old are you? This can be caused by a lot of things, and some would be more common for a young woman just starting her cycles and some would be more common for an older woman. Have you had any kids? Have you ever had a uterine polyp or fibroid? Are you taking any medications? How is your thyroid?
My suggestion is to see your ob/gyn, and maybe he or she might also refer you to an endocrinologist.