What Causes Severe Pain In The Buttocks?

Have had glut pain, more pronounced on left than right and often may radiate down leg but rarely below knee. Pain in center of glut and at insertion of hamstrings and that area. Have had massage and started when exercising but not exacerbated by exercise. Foam roller and massage help but temporary at best. Have mild to moderate osteoarthritis lower spine but low back not involved pain wise most of time. This has been ongoing for five years or more, not overweight, hike, backpack, very active. Have been diagnosed long ago w fibromyalgia but as stated am very active. Was doing exercise boot camps for several years when these glut symptoms began. Any ideas? YYYY@YYYY