It could be associated with the PMR, something called piriformis syndrome commonly causes buttock pain and soft tissue problems don't show on plain x-ray, they just rule out bony involvement. A physio MIGHT recognise what it is.
Always goes for the weak points! That place is where the muscle goes over the "sitting bone", the ischium to give its proper name. Could be bursitis or could be myofascial pain syndrome
That is strange, i am waiting for a new hip .i can feel the bone when im sitting or laying in bed. I have to move around till the pain eases , as you say it is just like sitting on a rock ! Hope you get it sorted .🙂
If it is piriformis syndrome there are simple exercises online to help relieve the symptoms. They are stretches or one is sitting on a tennis ball. They do help. I had piriformis syndrome and I did the exercises for a couple of weeks and it has gone now. Good luck.
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