I started doing hot yoga twice a week about 2 months ago. I started to notice a bit of tenderness around my tailbone, but I was also a bit sore in other places too, so I just figured I’d been stretched out.

Well 2 months later it’s now only my tailbone that regularly hurts after. I’m not in agony, but it’s almost a constant tenderness. It feels like I’ve spent too long sitting on a hard floor.

Any ideas what pose or what’s doing this?

It sounds like you could be one of the many people with extra tail bone vertebra, it just means that your tail bone will stick out a bit more when you sit down, take two small towels into class with you, fold them and put one under each butt cheek, this makes a runway for your tail bone, then it won't get bruised.

I have many people in my classes that I do this for.