I had been about three weeks late on my period, however I didn't think much of it because I didn't feel any different and believed it was my birthcontrol's doing. I had begun feeling crampy just as I do with periods however no bleeding then later throughout the day I did begin to bleed but as the pain increased so did the flow. I believe this to be my period until the pain got worse and became untolerable. I went to the er but didn't mention my suspisions of being pregnant because I was menstrating or at least thought I was. The said it was a uti and sent my home. However this morning two huge chunks of some kind of tissue came out of me not the normal little dark red clumps that discharge with a period. Is it possible I had a misscarriage or is it possible that it may have been the lining of my uterine still intact? The two clumps of tissue were tanish pink with black spots both as long as my index finger

It's possible that you had a miscarriage, it's also possible that you're just having a very heavy painful period. There's no way to tell you which is going on, we can't diagnose over the internet.

I would suggest that you see a doctor and mention that you think you might be miscarrying.