As a person who's cracked ribs at least twice -- look, it's easy to do, okay? -- I'd say you'd probably remember a specific incident where you went crunch if that were it. However, it could be an intercostal muscle that's angry at you, which feels a lot similar. I had that a few weeks ago after yoga, actually, because I overdid the twists after havingoverdone them at home the night before.

Anyway, I don't want to diagnose, but maybe you don't want to go to dance class and jiggle a bunch... maybe go to the gym and just do some very gentle stretching instead?