You could be pregnant. take a test as soon as possible.
How long have you been on this pill for? It could be breakthrough bleeding, meaning the pill isnt the right one for you anymore and you may need to try a new one (very common when on a lower dose pill for a long time.) Also, have you been taking it same time, every day, etc? That could be playing a factor as well.
I would definitely say pregnancy is a possibility. Take a pregnancy test as soon as possible so you can find out whether or not you are. This is extremely important because if you are pregnant and keep taking the pill then it might cause you to miscarry.