What might cause me to start my period before the end of my first birth control pack?

Break through bleed: On the pill or other causes your gyn can do an exam, blood work and an ultrasound, at your age you should be on the lowest dose possible, if you do not want fertility anymore you may be better off having an office tubal like I do on my patients and then after that if you don't want periods anymore which are worthless if you don't want to get pg, then you can have an office endometrial ablation.