There are a number of things that can cause your period to be off track. Since you are on the birth control pill, this could be the main cause. Other causes can be stress and diet, among other things. The hormones that are in the birth control pill can cause breakthrough bleeding, as jrb said.
Even if you have had a good history with the pill as far as your menstrual cycles, you can still have irregularity from time to time. It shouldnt be anything to worry about right now, but if it becomes very bothersome you can see a doctor about what you can do.
