pregnancy ?

So, I've been on the depo shot for almost two years and have never bled unless it's the week before my shot. I have sex with my boyfriend and don't use any other type of protection. I also let him cum in me, but I always get my shots a week before their due date. I had pink discharge about two weeks ago, and when I wipe, there's a really small amount of pink blood-like discharge occasionally, but not enough to wear a pad or really stain my underwear. Do you have any ideas as to why this is happening?