Violent Shaking While Fainting during blood test?

about a time ago, i got my blood drawn. i was a bit nervous because the lady taking my blood, took forever to even put the needle in my arm. when she did though, i fainted. my parents said i started shaking violently and my eyes rolled back. when they told me i fainted, i didnt know i had fainted, i even opened my eyes 2 times and saw my parents touching my face. i was going to tell them i was fine but as i was, i blacked out again. afterwards i was just crying and felt a bit tired. i probably fainted because of me being nervous, but why the violent shaking?