Hello and welcome to HCM,
Passage of clots during menstrual cycles is not uncommon.
The menstrual flow varies in different individuals.
In some females the flow is quite severe with passage of clots especially during initial days of 
menstruation whereas in others the flow is moderate.
Lower 
back pain is also common during menstruation.
However, the lower back pain is not due to 
kidney disorders but it is due to dysmenorrhea or the pain that accompanied menstruation.
If the symptoms are troublesome you can take medication to control the pain and also to reduce the menstrual flow.
I suggest you to consult your gynecologist for clinical assessment and prescription of pain killers and medication to control the menstrual flow.
At the same time, you are prone to develop 
anemia due to profuse blood loss so a hemogram needs to be done.
If anemia is detected, you can get treatment for anemia.
Thanks and take care
Dr Shailja Puri