Hi there, it can mean a variety of things. Its generally due to a change in hormone levels, which can be caused by many things.
Are you concerned? If so, why? What are you worried it might be? As further background information may help provide better advice. I don't want to scare you by rolling off possibilities.
You can find further information yourself at: http://www.nhs.uk/Conditions/Periods-irregular/Pages/Treatment.aspx
If you have concerns please seek professional advice
I would recommend going to see a doctor.