Sudden nausea and abdominal pain a few minutes after donating 400cc of blood, relieved after a lot of vomiting may be due to decreased blood volume, hypoglycemia, lowered blood pressure, insufficient blood supply to the brain, etc., and you should consult a doctor to find out the cause of the disease. 1. Reduced blood volume: blood donation will reduce the amount of blood in the body, the blood is mainly water, the reduction of water will make the blood concentration rise, that is, blood concentration, affecting the nutritional supply of brain cells, resulting in nausea and abdominal pain, relieved by a lot of vomiting. 2. Hypoglycemia: after donating blood, the glucose in plasma will also appear to be reduced, if the glucose content is significantly lower than normal, when hypoglycemia occurs, it will lead to nerve function disorders and lack of oxygen in brain tissue, and dizziness, nausea and vomiting will occur. 3. Decrease in blood pressure: blood donation will make the blood vessels contract, the amount of blood in the blood vessels decreases, the blood pressure decreases, resulting in dizziness, nausea and vomiting. If there is a sudden nausea and abdominal pain after a few minutes of donating 400 cc of blood, which is relieved after a lot of vomiting, it is recommended that the patient should consult a doctor in time, so that the doctor can clarify the cause of the disease and give targeted treatment.