Passing tissue

I got the depo January 12 I was due April 6 this is my fist time getting depo. I decided not to get it again because I want to try to conceive. Yesterday morning I noticed tissue on the tampon so I went to Walmart to get a douche to clean it out. And when I did a lot of pieces big and small came out. I'm not pregnant so it not a miscarriage. Since the 31st of march I've been spotting light blood and mostly dark brown with slight cramps here and there. I'm wondering why this happened. Am I forming a new lining from the progesterone in the depo. What's going on?