What Could Cause Tissue And Black Secretion During Periods Instead Of Blood?

Hey, so instead of my normal period blood, I have a black period with tissue coming out of me. It has been going on for about 5 days now, and it decreased itself, so it's pretty much done. I was wondering what could be the possibility of this? Two weeks ago I didn't have sex, but my boyfriend was rubbing it against me, (i had my underwear on, he did not). I got really scared and took plan b the next day but I am still wondering if I could be pregnant?