Brown spotting caused by Plan B pill?

I had sex (for the first time) 6 days ago. He did not come in me, but there was probably pre-come and we freaked. We got the Plan B pill a couple hours later that same day. 4 days later (which was 2 days ago) we had sex again irresponsibly. Again, he did not come in me, but there was most likely pre-come. Today, I thought I got my period (it's due in 7 days), but it was just a brown discharge. I'm very worried that this may be implantation blood. I have read that Plan B can cause this brown discharge. Please help.