Blood tests

I am a lab tech who collects blood. Fainting after a blood draw is not uncommon, especially for people who are fasting for their lab tests. Sometimes it is because their blood sugar is low. Sometimes people faint because of a vasovagal response to having their blood drawn. If the sight of blood or the stress of having your blood drawn makes you feel light-headed, this is vasovagal. It's just your nervous system over-responding and has nothing to do with blood sugar levels.
Either way, if you tend to faint during blood draws, it's important to let the person drawing your blood know this. They can have you lay down during the blood draw to prevent you from falling and injuring yourself in case of fainting.