Gray Clumps? On BC Pill.?

I'm on birth control. But I missed one day and then had sex with my b/f. I took plan b the day after just in case. A week later I had my scheduled withdrawal bleed. It was normal other than there was a few gray clumps mixed in two different times. It was dark grayish. What could this be? Is that normal after Plan B? I don't think I'm pregnant as I did have a 5 day withdrawal bleed and I also took a pregnancy test a week after this withdrawal bleed and it was negative.