I think I cracked by tailbone in high school. That was the worst pain/discomfort I've had. It took a few weeks to really get better...and still randomly get pain for a while after that. Nothing that you can do for it either. Pain meds and a butt donut are about it.
I have, actually. Like yours, nothing at all to cause it. I don't have any advice or remedies or diagnosis unfortunately.....Other than the fact that after a day or two it just ....went away. No idea. But I know the pain.
I’ve had it too. Several times. Makes it feel like your tailbone and hip bone are detaching from your body. They recommended apply heat and drawing a warm bath to sit in, and pain meds. It went away and I avoided doing any exercises that would aggravate it