Birth control.... Yasmin?

Hi my sister is taking Yasmin and her bowel movements have been horrible since she's been on it. She says it's either like diarrea or little pebbles (sorry about that lol). Has anyone else had this problem taking Yasmin and if so did you switch to a different kind and if so what kind? What could the problem be? Would she need a lower dosage or something? Please help she's driving me crazy with all these questions I can't help her with. Thank you sooo very much for the answer!