I Am On The Depo Shot Due For My Next

I am on the depo shot due for my next one in a week and a half for the last three weeks I have been spotting some days heavy but not many about four times through this I passed large masses of tissue. Now the last 5 days, although wearing a panty liner there is nothing on them but every time I use the bathroom there is blood on the tissue. I have not experienced any cramps through this whole ordeal or any type of pain, but I am concerned about the blood when I wipe. Any thoughts to what could be going on would help.