On contraceptives. Having dark brown discharge with unpleasant smell. Not STD. What could this be?

I am taking the contraceptive pill called Marvelon ... I take this the sophisticated way , taking packets of pills back to back with no breaks whatsoever (this was introduced to me my a Nurse at the Clinic). For the past year & a half I haven t had any problems until now... For the past 3 weeks I have experienced dark brown discharge which hasn t got a pleasant smell, I ve been taking the pills as normal. Risk of an STI is zero as I haven t been sexualy active (I only really take to pill to prevent myself from having a period) but today I ve started bleeding ... What s going on!? -_-