If it is the first time it has happened then I wouldn't worry about it, just make sure you make a note of the dates in case it happens again later down the road. As Zuhail said it is caused by an inbalance in your body. Whether it is due to hormonal, weight gain or loss, exercise, or simply stress in your life. All of these can have an effect. Like I said just remember the dates just incase it happens again and just make an appointment with your doctor to see what is going on.
It is not normal but you don't need to be worry about. You are experiencing irregular periods. Irregularity of periods can be for many reasons. The basic reasons for this are; Stress, Medication, Exercise and Hormonal Imbalance. A women can get 2 to 3 periods in a month and some only have it couple of times in a year. You should see your doctor for check up and treatment.
Well I had the same thing with my daughter I got 2 periods in 1 month then bang I was pregnant now I have had the same thing happen twice I have felt kicks in my tummy in which I'm going to see a dr tomorrow due to the fact is I'm not pregnant