Welcome at HCM.
I have read your question completely.
It looks that you are having pain after your periods.
The after periods may be there due to changes in pelvic or inner abdomen region during & after periods.There is increased blood flow to womb & contraction of womb to expel inner products during menstruation
.These changes may lead to stretching of the ligaments inside pelvis.
There are also associated changes in the ligaments ,like relaxation of ligaments which changes the bio mechanics at hip region.This may be the reason for your back pain
& buttock pain.
At present you should do rest.
Take a good analgesic
.The indometahcin will be particularly helpful for you.
Take good diet,plenty of water.
Preparation of primirose oil has also been shown to good in some cases.
Hope this all will help you.
With best wishes.