What Causes Inter Menstrual Bleeding While On Depo Shots?

i have been on the depo shot for 3 years and have never had a period while being on it until now. I ve been bleeding on and off for almost two weeks. sometimes it s brown discharge and then its new blood. i thought it was coming to an end yesteday when things seemed like they had stopped until i had sex with my boyfriend last night and now this morning its like im on m perod again. im freaking out because i dont know whats going on!!! help??