What Causes Bleeding While On Depo-Provera?

Hello I am on the Depo-Provera, I've been on it for 6 months, have 10mg every 3 months or something like that. I am now experiencing what I think is period blood but may not be, so can you still have blood when on it, after taking it this long? My 2nd jab was I think at the end of April, thankyou for your time.