What Causes Missed Period And Brownish Tissue In Urine?

Haven't gotten my period since Oct 2013. I usually get my period every 35-40 days. Took several OTC home pregnancy tests in December & January. Some were blank and some were negative. For the past couple of weeks I've noticed on a light pinkish to light brownish pieces of tissue in my urine. And I've been having back pain. Could this be a pregnancy or miscarrage?