There are several causes of this. There are signs indicating a monthly cycle that does not produce an egg. Usually a period that comes with no warning, is out of cycle. This is often accompanied by heavy bleeding. Before you get care for this problem, it's helpful to keep a log of your bleeding.
Blood tests can help determine what is happening.
The womb lining may be tested to see if it has gotten the right signal from ovulation. If a problem with ovulation is found, there are ways to treat it. If there is a disease, such as a thyroid problem, treating the disease will help ovulation. Physical stress, as in severe starvation caused by anorexia, can affect ovulation. Only by correcting that problem will the body be able to work normally.