Hello and thank you for posting your medical question on E health Forum. I am glad I can help you here.
Irregular periods and spotting post-pregnancy is due to high levels of prolactin, which maintains breast milk production. It also prevents ovulation and these anovulatory cycles present in the form of irregular cycles.
The lethargy and fatigue probably could be due to anemia, post partum hypothyroidism, etc.
Your doctor has the blood tests to check for the same, and the the ultrasound of uterus to make sure everything is ok.
I hope this helps.