Spotting on depo shot

Alexandria
So I started the depo shot in September 2015 and have been on it since. my next shot is in March. I've been not having a period on it and I'm not pregnant. Not sure what this is. Is my body finally getting used to the effects? I had a baby in 2014 and restarted birth control in September of 2015. Could that be why the delayed reaction? I know I need to see a doctor but I can't afford it but only for birth control..