What Causes Irregular And Dark Brown Bleeding?

Hi, I am 28 years old and have always been regular with my time of the month and have been on birth control for 5 years. This month I started bleeding a week before my period is due. I have not missed and pills nor have I taken any late. Also, it's a dark brownish color. Just some concern since this would be the first irregular period I've had in 14 years. Thank you!