What Is Causing My Tiredness And Heavy Bleeding?

Hi, I had my periods on the 5/09/2011, which finished on the 9/09/2011. After that, I had unprotected sex on 14/09/2011, after which I took Postinor2, even though I took all the tablets once, which I have always done, if I take them. After exactly one week, I started bleeding and its been going on for 8 days now, the heaviest 2day, which had very thick lumps. To add on that I have been feeling very tired, until I just cant concentrate on anything. I am normally a very active person, and been exercising about 45 mins a day, and was thinking maybe its the exercise. I am now getting worried, caz am very very tired. Please advise.