I've got this but not as a result of the bike. I had a baby 5 weeks ago and think I've agrevated a previous break in my coxyx. I totally know what you mean when you talking about the pain getting up off a stool. On my bike though I only experience it on the trainer when I'm sitting up, well back, with my hands off the handlebars. Whenever I'm forward it does not hurt. I'm seeing my dr soon and was planning to ask about it then but suspect it will just take time to heal (like it did last time). But it seems like your case might be different as I certainly remember the events that caused the pain to start, could it have been something other than the bike that caused the problem, in your case, like a fall? Regardless I would get it checked by a dr as the others have suggested.