What Causes Inter Menstrual Bleeding After Taking BC Pill?

i had my first injection on january 4th 2011 after changing from the pill i was advised to take the last 2 pilss from my pack then 5 from the new pack which i did. I had a period on 17th Jan and started bleeding again yesterday. Although it is not like a normal period i have had to wear something is this normal and if so how long will it go on for.