What Causes Vaginal Bleeding After Getting Depo Shot?

Hi ive been on the depo shot for 3 months now and ive been bleeding the whole time i was on it and ive been having some brown discharge as well im thinking bout swtiching birthcontrols to the pill because of the bleeding .is there any other birth control that i could take to either slow the bleeding down or stop it for a couple of days because being on depo is no fun and its ruining my life. So please help me.