It sounds as though you could be starting the menopause, have you had any other symptoms such as:
- Hot flushes or night sweats
- depression, anxiety or tiredness
Of course there are other reasons why this may be happening, if you have been stressed, started a new diet etc. This can make a difference to your normal cycle.
My advice would certainly be to see a doctor, I really don't think it is anything to worry about but things like this should never been left untreated. You are still young to be starting the menopause but every woman is different - I like to think of us as unique :-)