You could have missed a pill or taken one late causing your period to start early. This is the only thing I can think of. Or if it's a new kind of birth control for you, it may not be the right kind. Talk to the doctor that prescribes it for you and they will help yo find a better kind.
Sometimes that happens especially when you first start taking birth control pills. It is called 'breakthrough bleeding' a side effect of the pill.
This has happened to me twice in the 9 months I've been on my pills, once in the 5th month, once this month. I've never missed a pill and take them pretty well on time and both of these times have been weeks where I've been under a consideral amount of stress. At first I was concerned but after talking to my doctor she said that it's completely understandable for your body to be unbalanced under stress. So if you're stressed and you notice you get spotting/bleeding (in the last week of the active pills) then that's most likely what it is.