What minimus said:
Intuition.....BUT you must figure out what it is that bothers you so before you lose your child.
My dad would not sign my marriage papers the first time I got hitched. He did not like my ex-husband, he knew the man was bad for me. B A D with capital letters. I did not talk to my dad for three years for refusing to sign my marriage papers. But in the end I admitted he was right. I knew it cost him many a sleepless night, but my dad would not sacrifice his ethics, no matter what the personal cost. He taught me a lot about standing up for what is right. I once got that "vibe" from a bunch of new friends who attached themselves to my son. When they looked at me, they looked through me. They had no respect for authority, in my opinion. I told my son they were bad news. He did not listen to me. They ended up stealing his car three times, and when he reported them, they threw a brick through our front window. Don't say much to your son about this girl while it is just intuition, but keep your eyes open. When you know WHY you feel this way, speak up. IMO