Some people ped out today at my school when they got the swine flu vaccine?

Today at my school some people got the swine flu vaccine. 3 people ped out and they were all in my grade. One girl ped out right when they stuck the needle in her arm and fell on the floor. A few people came up to her and when she woke up, she asked what happened. They told her and she ped out again! She got tooken to the nurse in a wheelchair, just like the other two. How could they p out just from a flu shot?