The worst cramps of your life? This could mean lots of things, including food poisoning (you said your sister has cramps, too) or serious injury. Get thee to a doctor or an emergency room if this is serious pain and, as you said, you don't know why. Go. Now. This could be very bad, or it could be something normal, but cramps by themselves are not enough for us to tell you what they are and someone really ought to look at you ASAP.
"Ooh, I'll settle down with some old story/About a boy who's just like me/Thought there was love in everything and everyone, you're so naive!"-Belle and Sebastian, "Get Me Away From Here I'm Dying"