Birth control help

Okay so ive been on birth control for 3 months now & obviously you supposed to go every 3 months to get another shot ... okay so my next appointment is the 22nd of this month , and my question is how do i know when my birth control is wearing off or like what happens? Cause lately ive been getting this thick white discharge or whatever it is and its alot of it . And im concerned . And i know i dont have any std or anything cause i dont really participate in those activities and if i do its with one person. I need help