Menstrual cycles abnormalities/changes can occur for a wide variety of reasons from the benign such as stress to more serious conditions like cancers. Generally, if there is any change in the pattern of your menstrual cycles especially if the change if persistent, it would be advisable to seek medical advise. Normal menstrual cycles do differ from person to person but it usually would be between 21 to 35 days.
Thus my advise would be to seek further evaluation with your doctor to determine the cause of the change of your menstrual cycle. He/ She will need to asks some questions like when did you start menses, your previous menstrual cycles, medications/ supplements (including oral contraceptive pills). Also whether there is abnormal bleeding between your menses or after sex. Some tests may also be ordered like hormonal tests, ultrasound and pap smear.