text: Wow, it could be all manner of things. Birth control pills/ patches can cause her periods to be irregular. PCOS (poly cystic ovarian syndrome)or an STD. Have her see a gynocologist ASAP. Please don't wait on this. Could be a sign of a serious infection that could possibly interfere with fertility. I don't mean to scare you, But I have to impress how important it is that she see a doctor