I was just sitting at the table. I stood up, not fast. I was hungry, even though I've been eating all day. I just ended my period today. I stood up, I felt like I was going to black out, and started shaking uncontrollably. My knees where shaking really fast back and forth and then I hit the floor. I have only blacked out once and it was years ago when I was under 13. I don't know why I would shake like that?