Why am i bleeding between periods?

i am 19 and have been taking the contraceptive pill (mycrogynom) for 2 years with no problems, except this month i have been bleeding quite heavily from day 10 in my packet. i am now on my second to last pill of the month and the bleeding still has not stopped.This has never happened before- (i didn't even experience breakthrough bleeding at first.) what could be the cause of this? i dont know wheather to go to my gp or wait another cycle to see if it happens again. help!