I solved the issue.
Last night, I switched my WTB saddle (only used once) off my mountain bike onto my road bike.

That race-lite (or whatever the name of it is) generic saddle was the culprit.

The WTB saddle was comfortable - felt a little wide in the rear but that's because I'm used to the other one, which is quite narrow. But the little canyon they put in there in the back was the ticket - it took all the pressure off of my poor tailybone.