What Causes Spotting And Solid Tissue Discharge?

I last had sexual intercourse on April 17th. I was fearful of pregnancy even though he withdrew the very last minute and ejaculate on himself. I tested three times the next two weeks and tested negative. I did have my period on April 20th and it lasted its usual 7 days. Now, I've been spotting brown blood for about 2 weeks now and it's gotten heavier in the sense that I have solid tissue discharged every now and again. I've stopped taking my birth control 3 days ago. A little help? The discharge smells fishy like I normally would on my period.