What Causes Irregular Periods?

Hi, my name is andy and i actually had a question for my girlfriend but she's too embarassed to ask a doctor in person. She has lately been having her period very irregularly. It happens probably once every week and a half or 2 weeks rather than the normal month menstral cycle. I was just wondering if this was any cause for concern or if it is just a phase. This has happened 3 or 4 times now. Thank you for your help.