HELP!

A little box used to pop up every time I went to itunes, it would ask if I wanted to sync my ipod with the other one that was also being used on my computor. I clicked okay accidently and now my music is on my sister's ipod and I have all this wierd music on mine I was wondering if anyone knew how to undo that or if you could undo it. If anyone knows please respond to this!!!!

Thanks,
idk