Stared spotting after 8 days taking plan b and sex

Last night i started spotting brownish colored blood. I don’t know if it’s because of the plan B since i took one last month and didn’t have any effects of spotting. So this morning i took a digital pregnancy test and it says negative. Idk if this is implantation bleeding and it’s just too early to detect since it’s only been 8 days or this spotting is caused by plan b. Please help