Stomach upset and nausea are common symptoms and indicate that your body is adjusting to the new vitamins and minerals you're taking. To ease this problem, make sure you're taking all supplements with food. Not only will this help relieve symptoms, but certain micronutrients and cofactors in food actually help increase absorption of nutrients in the supplements.
Your body should naturally adjust to your new diet within 1-2 weeks, alleviating symptoms. If not, please contact your physician.