What Causes Passage Of Bloody Tissue From The Vagina?

A while back, I had a strong suspicion I was pregnant. My period was three days late, which was a string symptom in it's own for I havnt had a late period since puberty. Buy before I could find out for sure, I suddenly got my period, and basically bleed out for a week with severe cramps and triple the usual amount of blood. I put it down as I either was never pregnant and my body just messed up for whatever reason it I was pregnant and lost the baby. This was a month and a half ago. This past 12 days I've been on a really long period, but wasnt concerned because I had just started depo prover, and expected my cycle to be messed up. But over these past couple days I've passed two hunks of what appears to be bloody human tissue, and am wondering if it could be from the possible misscarge or something else entirely, and is it something I should be worried about?