Side effect or birth control or no??

So I'm on the depo shot and I haven't had a period in 2 months but this month I have but its been very strange. I first was cramping horribly and bleeding alot more than usual. Then a large peice of tissue came out and the cramps stopped and bleeding became lighter. But now its like my period starts to look like its going to stop (the brown discharge) then I will start bleeding again! Nor heavily but still bleeding. Longest period if ever had. Is this normal?! Should I be concerned?!?!