I recently developed acute tailbone pain. To be more precise, it's pain from something just to the side of my tailbone and towards my backside. It's mostly on one side although when it gets bad it's clear that there's also some tightness on the other side. It feels like a muscle or a tendon is really really tight and can't move, and that can make it very hard to find any position that's comfortable even if my tailbone isn't touching anything. It developed over the course of maybe a month. Needless to say I'm unsure whether this is the same tailbone pain described above, but it seemed similar enough that I thought I'd mention it.
After a good deal of experimentation I found that calcium and small amounts of oral vitamin d seem to fix it. I currently take two pills of calcium hydroxyapatite and 4000 mcg of vitamin D per day, divided into four doses. This has made night-and-day difference--there's still a spot near my tailbone that hurts when pressure is put directly on it, but I'm able to move around without fear of hurting myself, and I can sit all day without causing that spot to hurt. This is only day 4 of this treatment and I'm hopeful that over time the tight spot will disappear entirely.
The weird thing about all of this is that I have used a vitamin D lamp for years now. I get a certain set of symptoms if I don't use it, symptoms I used to associate with vitamin D deficiency. But I haven't gotten those symptoms for some time (I've been using the lamp regularly), and I tried increasing my UV-B exposure before I tried oral vitamin D and it didn't do a whole lot. So it's strange to me that my body would need ORAL vitamin D when it appears to get all the UV-B it wants.
Incidentally, I also tried taking fairly large doses of calcium before I tried taking small amounts of oral vitamin D. While it helped a little, it didn't help a lot like tiny amounts of oral vitamin d helped.
@Gondwanaland does this line up with how you'd imagine a salicylate issue would show up?
Here's a website where a new mother mentions what appears to be the same kind of tailbone pain and her success treating it with calcium.
As for what caused it, nearest I can tell is that use of a small amount of activated charcoal every day may have caused a localized deficiency of vitamin D. Which doesn't make a lot of sense, but I'm trying to rationalize why I need oral vitamin D when it appears my UV-B supply is sufficient. It's the best guess I have, although I'm curious about Gondwanaland's salicylates.