Nystagmus, "eye wiggles" is a common side-effect of MDMA (ecstacy) and other related drugs. Your vision goes blurry as your eyeballs are shaking from side to side more rapidly than you can voluntarily move them. it's a lot like like shivering, a trembling hand or chattering teeth.
Like Webster says, it is rapid and involuntary, and it temporarily impairs your ability to see. Voluntarily moving your eyes back and forth is not really nystagmus as it is not rapid or uncontrolled, any more than opening and closing your mouth means that your teeth are chattering.
Under the circumstances, I found it weird and interesting, but not unpleasant. In fact, everything was pleasant right then. However I can see how it could be considered an impairment if you need to do something that requires being able to see straight, or if it didn't stop.
Some people are able to induce a little nystagmus in themselves by determined squinting.