Let me know how this helps.
If you aren't able to do the exercises, like you're in class, try squeezing your glutes together really hard, and try and "crack" or pop your tailbone. Aligning that articulation, (the "pop" actually comes from release of trapped carbon dioxide gas in the joint) will stretch a few of the vertebra above the pelvic articulation, and provide some instant, although temporary relief.
If this pain persists, see your physician.