I Took Plan B Last Sunday Morning After Having Protected

i took plan b last sunday morning after having protected sex on saturday night. we took it because i had accidentally gotten some of the cum on my leg when taking the condom off. i woke up this morning and had brown with bright red spotting and it has been going on all day now. i have had abdominal cramps, no sore breast, and nausea but i have had constipation and nausea on and off all month before having intercourse. i'm really scared the spotting could be inplantation bleeding. i took a pregnancy test last night but it was negative, and i heard it was too early to take. please let me know!