Have you ever heard of the Piriformis muscle ? it is located deep in the buttock.
It can cause really severe pain[sciatica]
https://www.webmd.com/pain-management/guide/piriformis-syndrome-causes-symptoms-treatments
There are several stretches for it, that really helped me....But if you decide to try one [them]....go slowly.
The one that seemed to help me the most is actually pretty simple to perform.
Sit in a kitchen chair, cross your right leg over the other....ankle @ about the knee of the left leg. Take that leg
by grasping it at the knee with both hands, and pull it toward the opposite
shoulder. Do it slowly and do not hold your breath. Do the same with the left leg , ankle @ the knee, grasp the knee & slowly pull toward the right shoulder. Remember to keep breathing.
Alternate legs , maybe just twice each, the first time. And try to increase the range of stretch as the muscle loosens. maybe 2-3 times per day, over a day or so it should help..........remember to breath!
I am not a doctor....This is just what I do......google is your friend.