Could breakthrough bleeding on the pill be something else?

I have been on birth control for over a year and a half, and for the past year on the same one. My period came on Dec 24 when it would usually come on the 23 which got me a little worried. It only lasted a day and a half but it usually lasts about 4 days. It was also extremely light. It is now Jan 7 and I have had breakthrough bleeding since yesterday. It is heavier then even my last period was. I am going to the doctor on Monday just for my 6 month visit but I am wondering what else it could be in case I should make an earlier appointment.