Hey,

Well if you feel the bone is that far up, it is definitely not your tailbone. I think seeing a physio is a most excellent direction, especially considering you have muscle atrophy (less muscle) on that side compared to the other. Inflammation tends not to be a big waster of muscle, rather nerve dysfunction of some type does. Whether this is due to muscles that have become extremely tight and are compressing nerves, or if you're getting compression on the nerve root, or if the nerves are having other issues, this is usually why muscle wasting/atrophy occurs. Sure, longterm inflammation, so disuse of a muscle can create an issue, but...can't find it here presently, I think you are still quite young??

Go see a physio. You'll have your concrete answers then. Here we can only speculate and give general advice, as we're not actually assessing you. Good luck!