A meal very high in carbs induces the release of more insulin than after a usual meal. Insulin helps the carbs to enter the muscle cells and fat cells more quickly. If the time between meals is long (which it usually is with these types of diets), the consequence is a fast drop in blood glucose levels, hence – hypoglycemia, which may then cause shaking or nausea. Another possible cause of shaking and nausea on an empty stomach is stress.
It's pretty common to experience nausea or abdominal pain when you're under immense amounts of stress, and doctors are often unable to find a physical cause of this pain and nausea, or shaking.