What Causes Heavy Periods While On Birth Control?

I have been on the birth control Sprintec for almost a year now. I have never missed a pill and I always take it at the same time everyday. My period usually lasts 3 to 5 days and I'll usually have one heavy day and the rest are light. This month I have been on my period about a week and half. Instead of it getting lighter, it seems to be getting heavier. Should I be concerned?