What Causes Constant Bleeding While On Cerazette?

Hello there,I am concerned about my girlfriend.She has been on the contraceptive pill, Cerazette, for a few months now but she has been bleeding for about a month. Somedays the bleeding is very heavy accompanied with cramps, and other days it is so light, it gives us hope that she will stop bleeding, only for it to get heavy again.We are getting worried about what it could be, so she decided to stop taking the pill today to see if things cleared up.Could this bleeding be a sign of pregnancy?!