HI, I Donated Blood Today Like I Have Done For

HI, I donated blood today Like I have done for the past 5 yrs. but today was different after I was done and the girl was ready to remove the needle I guess I blacked out and the girl said my arms were twitching and she couldn't hold it down by herself, I was feeling really weird and dizzy I had a hard time focusing on what she was saying. What is that called and why did it happen?