What you are experiencing is called a vasovagal response, a reflex that occurs when your body overreacts to certain triggers, like blood or needles. The triggers stimulate a nerve that then causes your heart rate to slow down and your blood pressure to drop. As a result, you may suddenly feel warm or light-headed, turn pale, or lose consciousness.
To get through a blood draw when you feel dizziness coming on, try the applied tension technique: Tense the muscles in your body for 10 to 15 seconds, then release for 20 to 30 seconds, and continue. This helps raise your blood pressure, making you less likely to faint. Ask to lie down, too — then look away so you cannot see the needle entering your arm or the blood flowing.
It is not entirely understood why some folks have vasovagal responses and others don’t. The reassuring news is that these spells are usually harmless — and you should not need treatment if they happen sporadically and are not interfering with your quality of life. But if your reaction is constant and so bad that you dread getting immunisations, medical tests, or surgeries, you should consider seeing a psychologist who can help you work through your fear using exposure therapy (in which you practice being in the presence of needles until you are desensitised to them).