What can cause me to have sudden leg weakness at martial arts class?

Several things: This problem mandates a real-time (face-to-face) meeting with a doctor. A history has to be taken along with a examination as well as labs and other tests. Only after all this will your doctor be in a position to tell you what's wrong and what needs to be done to help you.

See your Doctor: There can be a variety of causes, but what first comes to mind is a low back condition which can often be aggravated with high floors, high kicking, etc. See your chiropractor to evaluate for neurological deficits due to disc bulge or herniation, as well as ruling out anything in the abdomen/pelvis.