I've never heard of any side effects from Tegretol related to the menstral cycle. I'm on tegretol, and it's never affected me that way. She's still young- your daughter's body may still be regulating. When I first got my period at 14, I only got it once, and then didn't have it again until about 4 months later(it was awesome- I didn't have to deal with it all summer! lol

). It took about a year before I was regular.