It doesn't have to be. It is true that clots in menstruation may be more frequent in the first days of menstruation, when the bleeding is more abundant and, therefore, usually have a deep red or darker red color than the rest of the bleeding. Thus, dark brown or coffee-colored clots may appear at the end of the period, as the blood is "aged".
In general, clots in menstruation are normal and occur in many women. However, if these clots appear frequently, are large and/or are accompanied by symptoms such as heavy bleeding, pain or tiredness, it is best to consult a specialist, as they may be caused by an underlying pathology.