Depo shot...

I’ve been on the depo shot for almost a year now, next month I am due for a shot. I stopped having periods after my second shot and a few days ago, I had brown spotting/discharge and it only lasted a couple hours. Is this normal? I have heard brown spotting/discharge is normal after having your period because it cleaning out the old blood but I haven’t had a period in months! I also heard that it can occur when you are ovulating and it could also occur in early pregnancy but there is no way I am pregnant because I haven’t had a period in months... we don’t use condoms and he does cum inside of me so we do rely on my birth control.. has anyone experience this before? I have been having other symptoms like wanting to eat everything in sight, little bit of nausea but that was last week and only lasted a few days but I thought maybe that it’s almost been a year so my body is getting used to the shot. Last time I had brown spotting was when I was 5 weeks pregnant with my son but I can’t be pregnant, it’s like impossible since I don’t have my periods anymore so I don’t ovulate then. I also heard that once you have brown spotting that it means the shot is not responding well with your body but I don’t 100% believe as I have only heard of that once in my life. Help...