ive had lots of pain from my tailbone when sitting n when moving to standing from a sitting position. mine is to do with weak pelvic floor muscles tho. so perhaps urs is the opposite n its a bit tight or ur over stretching it when u stand. I know the physio picked up on me pushing my hips out when I went from sitting to standing. it did feel like something stretched when I did it, altho there was no actual sound.
can u get to a physio?