What Causes Excess Bleeding With Contraceptives?

I started taking the contraceptive pill in October. I took it for around 3 months continuously, without taking the sugar pill, i then experienced cramps (which i never get) and some bleeding, so after a week of this i stopped taking the pill. As soon as i stopped taking the pill the cramps went and i had a normal period which lasted around 5 days, which is normal for me. When i got my next period i decided to start taking the pill again, however now after 10 days i am still experincing a little bit of bleeding. I stopped taking the pilll again after 5 days into this period as i realised i was still getting bleeding. What is the cause of this?