Thank you for writing!
While we can't give you a definitive answer, your post-menstrual fatigue may be due to a lack of iron in your system. Periods can cause a lot of blood loss, hence why doctors sometimes recommend an iron supplement for women. A lack of iron due to menstrual bleeding can cause the kind of tiredness you are experiencing.
Try taking a once-a-day iron supplement and see if it helps - but give it at least one month to work. If you still feel the same way, talk to your doctor.