What Causes Severe Pain In The Buttocks While Lying Down?
Muscle relaxant and chronic pain medication like gabapentin may be added
Detailed Answer:
I read your question carefully and I understand your concern.
It is a little difficult to judge on the origin of the pain (whether related to muscles as pain doctor suggests, or a nerve/joint origin) without having some reports on imaging and surgical procedure details (if you have them at your disposal you may upload in reports section). Given the recent surgery which I assume would have addressed nerve compression issues I also am inclined to think that the pain is related to local inflammation and muscle spasms.
As to what may be done....of course physical therapy must be initiated, it is vital as it is the only means to achieve long term stability, drugs are only for temporary alleviation of symptoms. As for what to take to alleviate current pain, looking at you have already been tried the remaining options would include a muscle relaxant such as Cyclobenzabrine and a medication for chronic pain such as Gabapentin, pregabalin or duloxetin.
If those are not enough local shots of steroids and anesthetics would be the next step (though you've perhaps already tried them).
I remain at your disposal for other questions.