@pleonasm - If you don't feel better soon I would go to the doctor and get checked out. There's always a chance that you could have torn something during your lesson and usually when you tear something in that area it needs to be sewn up to heal properly.
This might sound a bit crazy, but another thing that might need to be considered if the person suffering from abdominal spasms is a woman, is that she may actually be pregnant.
It's possible for women to get quite late into the pregnancy and not realize what is going on. They think their abdominal muscle spasms are just cramps and that they've gained a little bit of weight lately.
If any pain in your stomach gets bad, I wouldn't hesitate to go to a doctor. You could regret it if you don't.