What Causes A Period To Be Prolonged?

I'm having problems wit my period. I'm 19 and have had my period since i was 11. It comes on every 21 days and stays on for a full wk. My last period ended March 18. About a week after that it came on again, very lightly at first. Now it's not as light but not really heavy yet and has been on for almost 2 wks. What's wrong with me?