Totally have that pain. Think its a combination of things. First I think when my shoulders and neck area are spasmed up it just pulls on those hips and pelvic area.

Second I think all the sitting and desk job I do doesn't help, plus my posture is not that good. When I get up and walk around or stretch on my foam roler its better.

Third I did badly bruise my tailbone years ago when I tried snowboarding for the first time. What can I say the ski instructor was hot.

Last my gyn did dx me with something called pelvic dysfunction. Its a very real thing and happens to both men and women. As we age the pelvic floor gets weak. The pelvic floor is very important in posture and aligment. I actually have a constantly spasm'd levator ani muscle that pulles on my tailbone. There is a great book called a headache in the pelvis that has helped me with this and given me exercises to do.