I had the same problem when I was about your age. My obgyn at the time said it was not abnormal but put my on birth control for about a year. This allowed my periods to even out with the additional hormaones provided in the pills. After a year I was down to only one period a month rather than 2 or 3 and have not used birth control again. (i am now 28) this may be something to talk to your doctor about at your next visit. It sure helped when I eventually worked up the nerve to ask mine.

I have read that sometime it is caused by stress or it could be a change in ur body but also it could a sign of endmitriosis not scare you if it prosisted I would go to the dr it just something I wont mess with if it hasnt happened before