As you were able to see in the diagram above, the menstrual cycle is governed by hormones. The fall of the hormone Estrogen leads to a decline in energy. This occurs during ovulation and then again when the menstrual period begins.
Heavier menstrual flow also takes a lot of energy and can lead to fatigue even during normal menstruation. Unfortunately, this is considered normal.
All in all, you can factor in two phases of slight fatigue during your monthly hormonal cycle. However, if you experience extreme fatigue and struggle to execute routine activities, you should talk to your doctor about the symptoms. It could point to other issues like an underactive thyroid or anaemia.