Hi,
Welcome to ehealthforum,
It is difficult to comment on the cause for the post-period back pain that you describe without examining you. The causes could vary from ovarian cysts, endometriosis, adenomyosis, PID, pelvic adhesions etc. The exact cause can only be made out after careful evaluation and further investigations. Visit your doctor/gynecologist at the earliest for further examination and investigations to look for the underlying cause. Treatment as appropriate (based on the examination findings and investigation results) would be started to help you control your symptoms. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately.
Take care.