or perhaps she is simply hiding her struggle over dogma and belief and what the world around her is telling her? She may be confused about how much she was attending your Christian group because she liked you or another group member. She may be confused about the parental role in faith (there is one whether we admit it or not). Also she may have been influenced by someone else whose rebelling - maybe a new boyfriend? Hey she's a teen and rebelling is what teens are best at.
Youth give her time and space: which is what God does. Be patient just as he is.
And by the way, if you knew more about us (forum contributors) you'd probably discover that many of us (certainly myself) have been through similar rebellious times. . . . and in my case protracted ones. God never leaves us alone though - even in our rebellions.