What Causes Prolonged Period While On Plan B?

HI, I have had my period for ten days now and I dont know what is the cause could it be due to taking the plan B pill or could it be because I have been having sex more frequently. I had it for four days then it seemed like it was over, then I had sex then when ever i worked out it came and now its heavy like when I ussally get it whats wrong with me?