I Call It "my Hip Going Out" But Not Sure

I call it "my hip going out" but not sure its my hip. It actually seems to come from low and to the side of my tailbone. I don't have to do anything specific to cause it to happen. When it does, it stops me in my tracks. I can't lift my leg without extreme pain. I have to drag it to walk. It can happen on either side. I do have problems with my tailbone sticking out. I have never been able to lay on a hard flat surface without putting my hands under my butt. It's happening more as I get older. I do seem to have a posture problem with sitting. Do you know what is going on?