Am I in the clear or it's Implantation bleeding?

Alyssa
I just finished my period this Saturday that past and I had sex on that same day. I took plan B on Sunday and then had sex on Monday. Today I woke up and used the restroom, but when I wiped it was a dark brown kind of blood. So I'm not sure what's happening or what that means? Plan B says I can spot after taking it but I took it Sunday.
****I don't take birth control and he used the pull out method