I had my period twice this month whats wrong?

At the begining of the month my period was normal and on time it started on the 3rd and ended on the 8th. Then it started again on the 13th which this has nvr happened to me b4 i have been having my period for 8 yrs now i am on birth control and have been for 4yrs and i didnt do anything different with it. so i dont understand why i would have my period twice this month. What could be wrong? Anything serious? Thnx