first period

My little sister is what I think experiencing her first set of cramps right before she gets her period for the first time. She's saying she's in a lot of pain and doesn't want to move at all, and she keeps throwing up. I also think it's maybe the stomach flu, but I don't ever remember being in so much pain from it. Has anyone else felt the need to throw up right before they got their first period? Or is she just sick?