I have had something like this happen with some goldfish. I'm not sure if it applies to clowns or not...
Goldfish have what is called a swim bladder, this keeps them balanced. Sometimes when you feed them floating food, they can suck in some air with the food. The extra air inside them can throw off their balance. So basically they have gas. Sometimes all you need to do is wait to fix this problem.
I would THINK that this could apply to all fish. That's just a guess. I'm sure someone will correct me in no time at all if I'm wrong.
Hope that helps.