t'aint natural, I have developed the same snag. In the past, I had no problem at 10K+, as a matter of fact in Asia I would go up there routinely to escape the 35ºC heat. Nowadays, I just can't do it anymore, and I know a few guys who are the same way. It doesn’t seem to bother me IFR for some reason, only when I look outside. Funny thing is I could spend all day with the door off, hanging out and looking down from 200’. I’m sure that would hurt just as much!
I had a theory that because I had spent so much time looking straight down from the end of a 100-200' string doing precision work, that my butt had become hypersensitive to very small aircraft movements. At altitude, with very little visual reference, I was feeling the ship move, but my eyes weren’t seeing it. I don’t fly much anymore, so I never really dug deep to get an answer.