What Causes On And Off Fatigue Post Blood Test In A Child?

Hello, my 7 year old son had his first blood test today. He found it very traumatic and nearly fainted and had to lie down for a while. When he felt better he was very hyperactive and happy, had a big supper and has felt fine. suddenly he has been very sick. He doesn t feel ill with it, just exhausted. Do you think it is a result of the traumatic time he had today? Thank you.