What Causes Buttock Pain While Sleeping?

I get pain in my left buttock after sleeping. I use the CPAP with oxygen. The pain lasts for about a half an hour when I walk. I have to use my cane and the pain is great. This started about 6 weeks ago. I sleep either on my right side or my back. I tried sleeping on my left side. Then, for 3-4 mornings I had no pain. And then the same pain came back. I m 70, 5 4 and 315 pounds. I ve been overweight all of my lift. I just had a colonoscopy and they only found 6 polyps, all benign. Neither Aleve nor Excedrin make any difference. I just started 10 mg Flexeril yesterday, 3 tabs daily for 10 days. So far, it hasn t helped. I don t understand why the pain is only in my left buttock. I m not getting any pain in my leg or my back or neck. My blood platelets are normal now, which were low until I stopped Zyprexa and was reduced to 15 mg Abilify from 20 mg. My email is YYYY@YYYY