I would also say, it might not be considered "a miscarriage" if there was not yet any embryo formed.
It could be tissue left from when you took the Plan B. My sister had a D&C for medical removal of a (deceased) embryo early in a pregnancy, and several weeks later something came out of her that looked like what she described as chicken skin. We speculated it was part of the amniotic sac or the early placenta, that had been left behind when they used the curette.