I have a 12 about to be 13 year old daughter who has been on Sing...

I have a 12 about to be 13 year old daughter who has been on Singulair now for quiet sometime, with a few breaks here and there. My question is considering she is 12 about to be 13 and is just starting to become a young women with PMS and all, how are we to determine whether or not the mood swings and insatiability is PMS or some weird side effect of Singulair? I don't necessarily want to stop her from taking Singulair considering the investigation is not over yet, but I don't want to continue putting her in harms way by keeping her on Singulair. Anyone have any suggestions?