A question about the depo 3 month shot? TMI warning?

So i got my depo shot the day I was suppose to start my period which was Ded 8,2011. I was 16 days late and now iam pretty much spotting. Not even enough to fill a pad or tampon. I could use the same pad the entire time but that would be yucky lol. Does anyone know if it is from the depo or something else. Oh ya I took a pregnancy test that day before my shot and it was negative. Anyone got any tips. At some points in the day my boobs and nipples will be very tender. any advice would be greatly appreciated thank you