I don't have CF, I am a mommmy of a little one with CF but I thought I would let you know that I've had this issue before. The last time I switched bc my period was out of whack for about 3/4 months and then everything got back to normal. I think its an issue with your hormones when you switch pills, especially if the levels of estrogen in the pills are different. Don't know if that helps you out at all since we have different health backgrounds but I thought I'd throw my 2 cents out there.