Before everyone gets super harsh on this guy... Remember there are other reasons they put girls on bc. Either for pms problems, or their cycle is off and weird, and many more. Honestly your best option would be to take her to they gyno. It could be a number of things. Some not bad some more serious. Could be as easy as changing her prescription. Or something to worry about more. Kids are gonna be kids. The more open and honest you are the more they respect you and end up not rebelling as much.