OK, so here are just some wild brainstorming some easy guesses to start off -

• Low glucose (some meds can cause this temporarily, but others can cause permanent metabolic changes, also could be due to the vomiting)
• Low blood pressure (again, possible medication related, but sometimes happens on its own)
• Reaction (even delayed) to some food, such as gluten or dairy
• Missing some nutrient for whatever reason - it is surprising how many doctors don't check it all. You might want to look into spectracell.
• water or electrolyte imbalance (again, especially if vomiting is involved, but can happen even without that)
• Migraine