What Causes A Prolonged Period While On Birth Control Pills?

I had my scheduled period about back on Dec. 16th, it wasn't like a normal period it was lighter and more dark in color. Now after having that I started bleeding again Dec .26th, It's dark, and bright red at times, it's like normal period but has been going on longer than it should. I am on birth control, I have taken a pregnancy test the other day, it came back negative but I can't seem to figure out why this is happening.